Hi @MichaelG and welcome to the forum.
I use a Fantom 07 as a controller in my setup (alongside a StudioLogic SL88) and do indeed mix sounds on the Fantom and those in GP split and layered on the Fantom and the SL88 and even controlling some Fantom sounds from the SL88 when it makes more sense to control it from a weighted action keybed.
I would be surprised if you couldn’t achieve the same with the MODX, but sometimes you don’t always gel with the workflow of an instrument.
You’ll find details of my setup here: Dave Boulden