While I don't use it with my FTM-400XDR, a headset and foot switch can be adapted to the radio with a minimum of fuss. I have a setup for my FTM-100 in my shack and it's compatible with the 400 as well.
If you wish to use a headset in a mobile application, then a single-sided headphone on the headset is suggested. Some states have prohibitions against covering both ears while driving a vehicle.