No. My previous MO6 had a tempo knob I used for this but today the suggestions were to first navigate to the screen with tempo then tap tempo (not great for tracking tempo during these kind of gradual tempo changes - or .. instantaneous either) OR to touch the tempo value and use the DATA DIAL to move the tempo up/down.

You have to manipulate tempo directly.

I believe "Tap Tempo" was added as a MIDI Foot Controllable feature in one of the OS Updates. Check the Supplementary Manuals. IIRC you can use a Yamaha "Sustain" pedal as the Tap switch... which IIRC can be an FC4A or an FC5. I recommend the FC4A.

My experience from playing Guitar (usually controlling Delay Times) is that a Tap Tempo "Pedal Footswitch" is the ONLY way you can keep up with the Drummer... and a good Drummer can "communicate" the rate of change of tempo, allowing the rest of the band to follow.

EDIT: I'm not sure "Songs" will respond to changes in the Global Tempo of the MODX... they are just Audio (.wav) files... but I've never tried.

However MIDI Recordings and Pattern Sequencer will respond to changes in Global Tempo (therefore Performance Tempo). And so will ARPs. And as mentioned above, the Global Tempo can be controlled in Real Time with the Tap Tempo feature.

Is there a way to assign one of the sliders to change the tempo of a midi file recorded as a song. What I've done s far is recorded a backing arp part on ch1 and in the middle of the song the tempo slows down and then gradually increases to the end. Im trying to keep the tempo close to the speed of our drummer. I know it can be done with the dial but would be easier if I could just keep bumping it up with a fader or maybe even the super knob

A Slider or an Assign Knob (0-127) or even the Super Knob would be insufficient to access all TEMPO values _ There are 295 different BPM that can be selected for TEMPO. The Data Dial will increment by single unit steps giving you access to all 295 potential values for TEMPO (5-300bpm)
But trying to resynchronize an arpeggio to a drummer that cannot hear that arpeggio is like the ultimate in impossibly frustrating tasks -- please don't spend your time doing that...it is a full time job...

Antony is correct above, you can use a dedicated Foot Switch (momentary type FC4/FC5) as your TAP TEMPO - you do not have to select any specific screen to use this...
[UTILITY] > "Settings" > "MIDI I/O" > find "FS ASSIGN" > set to "TAP TEMPO"

But again, this is frustrating and a fulltime job. It would be far easier if the Drummer could hear your arpeggiator and play along with it. Or you can use a clock source that can be sent to both the Drummer and to your MODX via either audio or MIDI

Alternate (Better) Solutions
The larger subject of sync'ing technology to live musicians is discussed in the thread linked below... Although the discussion is for the MONTAGE, most of it applies to the MODX.
It includes a discussion of using the MIDI SYNC = A/D Input... but most importantly it discusses the real world reality of trying to match the technology's tempo to humans (when we know intuitively that it would be far, far, far easier to have the humans chase the technology's tempo)... We all know this - so far, technology cannot hear (this has not been foolproofed quite yet, anyway)... The Auto Beat Sync function can work but it requires practice if the source is human (like most things musical) and is by no means foolproof - when the tempo source is human and tempo detection is dependent on audio input.

If, however, you feed an automated tempo to your MODX via A/D In (or via MIDI IN) and that same source can be fed to your other band mates via a good monitoring system - then you have a great chance of locking in with your Tempos. This is how it is done when large tours have to sync to external musical sources - a good MONITORING system - "...this is the way!"

Please see this thread for that discussion:
Link -- Using External Tempo Sources with Live musicians

We'd be happy to help you sort through the options - but you know the best way is for humans to do the thinking and adjusting. Technology's strongest asset in all of this is - it can provide rock solid tempos - you just need to make it the CONDUCTOR for the entire band.

Thanks for the info. All of this is rock solid information and much appreciated.