Is it possible to use variable wing geometry on a fifth generation stealth fighter?
Of course it is possible. But it does not improve the outcome.

Swing wings were a fad of the Sixties. They never fulfilled their early promises, because the actuation mechanism is heavy. Variable sweep is only justifiable when the aircraft will fly both sub- and supersonically for extended periods. It turned out that the part of most missions which is spent at supersonic speeds is a lot smaller than anticipated in the Sixties. Just take the Rockwell B-1: Designed for supersonic penetration (B-1A) it became a subsonic low-level terrain following strike aircraft (B-1B).

For stealth, edge alignment is an important design principle. Making wing sweep variable is to throw this overboard, compromising the whole strategy of stealth. Also, the best stealthiness is only possible in subsonic flight - airframe heating at supersonic speed will broadcast the aircraft's presence to everyone with a low-cost infrared sensor.