For porting these, I can't really imagine a way of this being possible out of the sky.

What I think the approach is, is to Re-map the rigs to another rig. So you map the bones and link them to other bones, similar to how you map the rigs in VR Chat.

I am not sure if this here works https://github.com/Mwni/blender-animation-retargeting

or this https://www.youtube.com/watch?v=qnJFOSisDLQ

Apparently it's called Animation Retargeting.

This video here at the end shows me something reasonable that might work:

https://www.youtube.com/watch?v=Op0AvUChNTo

Here you map the bones, so that's what you'd have to do. And this means that you can do any game with this... If you ever saw animations in some game that is from some other game, and you know that the rig is different, I am pretty sure that they took these approaches. Or maybe they didn't lol. I don't know how popular this method is.

Ah yeah, the other method is to re-do the rig so it matches with the animation, but that's dumb if we are thinking about games to games.