WHY would you want to, in the first place?
also, what have you tried?
Typically, I export the scene to Quake format, then convert to dxf and bring it into FSDS using the CVA converter.
Caveat, some gmax models are complex and the converted dxf file will CHOKE the converter.
I did convert Milton Shupe's Beech 18S - in "chunks" - I created separate fsc files and then merged the files into one final "chunk"
No textures, no animations, all parts with their axis at 0,0,0 ...... only the most masochistic, True Luddite would undertake such a quest..!
I usually reserve that porting back and forth between FSDS-gmax->fsds for specific parts I want to bend, fold, spindle, and mutilate, like booleans or shape merge...