You mean port flex to 4.21? Yes it’s possible. You would need to know some c++ and the UE4 engine changes, if you follow along this thread here it sort of explains it:
https://forums.unrealengine.com/community/general-discussion/24447-nvidia-gameworks-integration?p=1568578#post1568578
The newest working flex was ported by 0lento for 4.20. You can see all his porting/custom work here: https://github.com/0lento/UnrealEngine/tree/4.21-GameWorks
Keep in mind he has flex 4.21 wip listed in his git but he only put it up so that someone else could attempt to finish the work needed.