Yes and yes. You can - but how well and simple this is depends on what you have on the other end which is going to be the master controller while using MODX as a slave.

MODX, out of the box, is in a mode where each PART is mapped to its own MIDI Channel. PART 1=Channel 1, PART 2=Channel 2, and so on. For the master controller to correctly serve as a master controller while MODX is in this mode (Multi Channel) - you need to have a master controller that will transmit note-on information for each channel you have PARTs. The maximum amount of PARTs in factory Presets (other than GM Init) is 8 PARTs. So your master controller would need to transmit on MIDI channels 1-8 (simultaneously) to cover all possibilities. Typically this is referred to as "zones" (by Yamaha and the rest of the industry). So if you have an 8 Zone keyboard, like many Kurzweils, the Roland RD 2000, Korg Kronos, or a minority of other keyboards, then you're OK using the default Multi-Channel mode. For the rest, there's two ways to accommodate.

The easy way to accommodate for an external master controller with less than 8 zones is to switch MODX into Single Channel mode (under [UTILITY] "Settings" -> "Advanced" and the "MIDI I/O Mode" parameter). Set this to Single and you can pick a transmit/receive channel which matches the MIDI channel your external keyboard is set to. This does "flatten" MODX and removes some possibilities of externally targeting specific PARTs for modulation - but it's the easiest way to make this work.

The hard way is to place something between your master controller and MODX which will take say one zone of MIDI output from your master controller and replicate those MIDI messages to all 8 MIDI channels. Something like Camelot Pro (running on an iPad, Windows Surface/Laptop, or Mac laptop) can do this. They have a demo of using a zone limited keyboard - the Yamaha MX series - to target all PARTs of a Yamaha Montage while the Montage is in Multi-Channel mode. Not something you could do without the software between.

That covers your keys. The parameters has a similar answer. There are a number of parameters you can get to through CC. The list is longer if you can keep the MODX in multi-channel mode. You could target the generic MIDI CCs (such as cutoff, release, etc) or you could target the PART-level assignable knobs (if your keyboard is in Multi-Channel mode) or you could target Superknob and program superknob to control multiple downstream parameters in response to its movement through external MIDI (Superknob can be assigned to a CC). You'll have to check what you can do with your master controller in order to send CC messages. There is less standardization here - so your mileage may vary.

Actually, I believe the answer to question #1 is no, unless you add an external device. For direct connection, your external controller has to have a standard 5-pin DIN MIDI jack, I don't think MODX can act as a USB host. There are devices that can do the conversion, but it's a bit clumsy... they need their own AC, and they may not work in all configurations, and they're not as cheap as jus buying a USB or MIDI cable, but they usually work. (Or you can plug both keyboards into a computer or iOS device.)

You're right - I totally glossed over the _USB_ MIDI part of this. If your external controller does not have a standard 5-pin DIN MIDI port (or some translation cable to provide it) - then you cannot plug in USB-only controllers directly to MODX.

My answer assumed 5-pin DIN which is certainly not how you framed your question. Apologies for the oversight.

If your External Controller only has USB connectors, you will need to connect through the MODX “TO DEVICE” Port (as of version 2.00 external MIDI can enter (only) via the TO DEVICE port).
If your External Controller has a standard 5-pin MIDI connection, you can connect it directly to the MODX (set MIDI I/O = MIDI)

I fully understand the need to approach certain synth sounds from a non-piano weighted action (solo leads). You can configure your setup quite nicely... even setting up Performances that allow you to switch between keybeds within a single Performance...

In other words, you could have your preferred weighted action program when you played the MODX8 keybed, and your solo sound when you played on the synth action keyboard... all from the same Performance!

If you must connect a USB controller directly to a MODX, here are some possibilities for converting the controller's USB Out to the MODX' MIDI In:

http://www.hobbytronics.co.uk/usb-midi-converter

http://www.kentonuk.com/products/items/utilities/usb-host.shtml

https://www.excelvalley.com/product/midi-usb-din-converter/

http://compasflamenco.com/midi-c-3/usb-host-midi-2-p-6.html

https://www.thomann.de/gb/mode_machines_cerebel_usb.htm

https://www.thomann.de/gb/miditech_usb_midi_host.htm

Some great suggestions above. I've been looking into solutions like this and have come across the iconnectivity mio range. It looks useful in that it appears you can save user defined routing as presets that can be recalled without a computer. I'm guessing (the manual suggest so) that you could assign the channel 1 output of an external keyboard to appear as multiple channels simultaneously, e.g. 9-16 inside the MODX. This may be appealing to people who don't want to use computers on stage.

One external controller could be playing 8 channels simultaneously and you could mute certain ones using scenes. Useful for single instruments that have been stretched across multiple midi channels.

Its a shame that you don't get the USB host function on the smaller mio2, so if you need that then mio4 has USB host but is expensive.

I know this is a bit off topic but I'm currently trying to set up my CP4 and MODX6 to work together, its really quite a head scratcher to overcome the limitations of un-mappable midi channels on both instruments and the fact everything is sent on port 3 because I need USB midi mode to have my tablet control program changes when I switch sheet music.

"Working together" doesn't describe if you're using MODX as the master or the CP4 as the master controller.

Using CP4 as the master (CP4 piano keys and controls used to operate MODX):
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If your MODX Performance uses 4 or less PARTs - then you can leave MODX in Multi-Channel MIDI mode ( [UTILITY] "Settings" -> "Advanced" paramter "MIDI I/O Mode" ). Then use zone control on the CP4 to set Zones 1-4 note limit to full keyboard and MIDI channels 1-4, each zone a different transmit MIDI channel. Since there is a possibility your Performances will use more than 4 channels (probably up to 8) - then it would be best to place MODX in single-channel mode (same utility menu and parameter, set to "Single" ). With the CP4 NOT in master controller mode (zoned) - the MAIN Part should always be transmitting which is fixed to channel 1. So while MODX is in single-channel MIDI mode, ensure the MIDI I/O Channel is set to channel 1.

Using CP4 as the master, under "Controller" - you have options for what MIDI CC the expression pedal and other other pedals send - which can be matched to MODX's.

Using the MODX as master (MODX piano keys controlling the CP4):
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You have more flexibility using MODX as a master controller. Each PART, when turning Master+PART Zone Control ON on MODX can set the transmit MIDI channel for each PART. You can access the main, layer, and split channels (ch 1, 2, and 3) of the CP4 and set any of your PARTs 1-8 to target those CP4 functions. Or you can leave zone control off and PART 1 will target main, PART 2 will target layer, and PART 3 will target split. If you do not have 3 PARTs turned on in your Performance, you can add 1-2 PARTs and set "Int SW" (internal switch) to OFF for both. This will prevent the PART(s) from triggering the internal tone generator and will only be used to send MIDI to your CP4.

Thanks for the reply, I think I might start a new thread so I don't hijack this one as it didn't seem so straightforward when I was trying to get it working.

Using CP4 as the master (CP4 piano keys and controls used to operate MODX):
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If your MODX Performance uses 4 or less PARTs - then you can leave MODX in Multi-Channel MIDI mode... Since there is a possibility your Performances will use more than 4 channels (probably up to 8) - then it would be best to place MODX in single-channel mode

Important caveat: If you take the second of those two approaches, the MODX itself will be limited to playing just one Part, unless you want its keys to exactly duplicate what you're triggering from the CP4.

1. I have connected a keylab essential and alesis strike pro to the MODX via usb and they both trigger sounds - nothing else to do than just plug and play.
2. I have that question as well - can I ceate a map where the drums on MODX can respond to alesis strike's CC4 continuous midi of the hi-hat pedal? That would really be good if possible.