I've recently spent entirely too much time trying to figure out the M-7s. You can walk one into a cryosleep casket if you want. You can't force it into an upgrade bed under any circumstances, and it seems to starve to death if left unoccupied in a bed.

Best solution I've figured is to shove a VX3 chip into your marksman and just permanently keep them in Control mode and connected to the M-7 unit. It'll spend most of its time "idle", but at least it won't starve to death--and with a VX3 you can also connect to a T3 or T4 surrogate so that you aren't missing out on the colonist's other functions. (Plus, as an added bonus, if the surrogate gets gunned down then you can just build a new one.)

I made a room just big enough for the m7 pod. After it gets downed I have one of the colonist take it to the pod then disallow the door so none of them tries to take it out.
When I need it again I just allow the door and have someone connect to it again.