So I tried and it works 
: once set up with the right CC’s you can use the X/Y control pad of ModStep as a joystick to control one or all the tracks of the MnM.
You need to create a control template in ModStep (as described in the documentation of ModStep) including the PitchBend and CC 1 (Aft) & 2 (Breath), and configure those as per the X/Y pad of the MIDI channel used to control the MnM.
On the MnM, configure the same joystick settings on all 6 tracks (which is made easier using the copy-paste feature)
If you prefer you can use different settings for each track’s joystick - I personally find it easier to control the same parameters (eg. filter base and/or width, EQ settings, amp decay, whatever) on all the tracks to achieve a Crtl-Al behavior and know what I’m controlling exactly.
If you want to use the joystick to control one track only, you can set the MIDI channel in ModStep to the Auto-Channel number.
If you prefer to control all the tracks (which is my case), set the ModStep MIDI control channel to the Multi-trig Channel of the MnM.
(You can find these MIDI channel numbers somewhere in the Global menu. Not sure where at the moment because I’m not in front of my MnM)
Voilà
So far I just briefly tried using my finger on the X/Y pad to control the 4 directions at the same time. Maybe it’s easier to draw the actual CC envelopes in the envelope section of each CC.
If you have a hardware MIDI controller with a X/Y pad that enables you to assign the CC’s you chose to the X and Y axes, I guess you can use it as a joystick too