You don't. Or, well, you could set it up as a Studio model with FK, but-
The only real way to go about it is to remake the body in Koikatsu and port parts of the character (clothing, hair, body textures) over as needed. You can also port over the head, but in all seriousness it's gonna end up with some jank, especially if you intend to use it in the main game or Free-H.
It's gonna be time consuming so uhhhhh good luck I can't help you there, try to find some guides :^)