Is it possible to reenlist for MFF if your unit has slots available? and if so is it solely up to the commander?

Since it's an ASI, you can reenlistment for it under Option 3, Army Training Reenlistment Option. You must meet the qualifications below to be able to attend.

ASI-W8
Title: Special Forces Military Free Fall Operations
Proponent: USAJFKSWCS
a. Description of positions. Identifies positions requiring participation in special military operations requiring free fall parachuting. (MOS 92R only, TDA positions at the Airborne Board will be identified with this ASI.) (MOS 31C skill level 1 only, MTOE positions in the 75th Ranger Regiment Recon Platoon will be identified with this ASI.
b. Qualifications. Must successfully complete the Special Forces Military Free Fall Parachutist Course, Ft Bragg, NC.
c. Restrictions. For use with MOS 11B, 11Z, 18B, 18C, 18D, 18E, 18F, 18Z, 25C, 25U, and 92R only. Closed to women.