Is it possible to connect an external midi keyboard to the MONTAGE 6, and play though it a different sound than the sound that is currently played through the MONTAGE keyboard?

Yes, it is possible. The MONTAGE has a "MIDI I/O Mode" setting called "Multi" which allows the instrument to Transmit and Receive on multiple MIDI Channels. When set to "Multi" the PARTs 1-8 can be played simultaneously by the MONTAGE Keyboard by simply setting the "KBD CTRL" icon to ON for those Parts, any other PART (KBD CTRL inactive) can be played by selecting it directly. Your external keyboard can play any MONTAGE Part by transmitting IN on the corresponding channel. For example, if you set your external keyboard to transmit In on Channel 16, for example, the Instrument you place in Part 16 will sound in response to the external device.

You would use the MONTAGE Zone Master = ON, this allows you to send controller info to your external Synth from your User Performance. Dedicate one Part (slot) of the User Performance to send information to Bank Select/Program Change, Volume, Pan, etc. If you do not want the MONTAGE keys to trigger the external keyboard simply use a slot not set KBD CTRL... this will allow you to send the Bank and Program Changes without triggering the external synth's tone generator.

The ZONE MASTER function is found [UTILITY] > "Settings" > "Advanced"
Set ZONE MASTER to ON
Set the MIDI I/O Mode = MULTI

Then in the Performance you are going to use, select the sound(s) you want to control via MONTAGE in PARTS starting with 1 through 8. Remember, up to eight can be set to be played by the MONTAGE via activating "KBD CTRL" (green) icon on the HOME screen. You can designate any Part not under KBD CTRL to act as the "external Part"

The "external Part" slot must contain a "dummy" MONTAGE Part which you can deactivate by setting the INT SW (Internal Switch) to OFF... this will prevent the Zone from triggering the internal Part but let you apply all the controllers for that Part to be routed Out on the MIDI channel that you choose. You can set the MIDI Transmit Channel to any of the sixteen channels to match your external device.

Within the Performance you can find the Master Keyboard functions as follows:
From HOME
Press [EDIT]
Press the [PART SELECT X] button, where X is the Part slot you choose to delicate for the external device...
It must contain an internal Part, which you will opt to set the INT SW = OFF (deficating it for external use)
Touch "Part Settings" > "Zone Settings"
Zone = ON
TRANSMIT CH = to the channel of your external device to receive Bank and Program message.
Set the INT SW = OFF ( this prevents the "dummy" Part from sounding but lets us address the external device)