Just a humble opinion, but isn’t the FM already pretty simplified.
All of the sound is generated on chip, and it seems that as is would be a great audio source for a modular. I guess the complicated bit is the controls you use.
If you want to control pitch etc. I believe it would be a case of requiring an AIN module that you could assign to certain parameters in the FM program.
It would definitely be possible. All it really requires is a little customisation of the firmware, and a number of DIN and AIN boards, (maybe some DOUT’s too). I can’t help you do it, but do a search and you’ll find all the info needed.