Money. Lots of money.
As far as I know you'd first have to turn it into a 4 door hard top, as the 2 door cab isn't long enough to mount a turret bearing (or if it is there is nowhere to stand properly due to the engine doghouse being there)
If it's not made for the HMMWV, then you'd likely need to have a custom B-pillar fabricated, and possibly a custom C pillar, though that's less likely.
Then a heavy enough roof and support for the turret bearing. If it is the same actual bearing turret as the HMMWV can use, then it's just the correct roof and supports.