How about an in game system for this?

Signing a free agent already does the claim system. If you're talking about claiming a player from another team that's not there... I've kind of left that up to the leagues to handle manually since there are so many possible rules around it.

Well, that’s the point, to have an in game system so it’s not shoe-horned in out of game. Just go with what the NFL has, or some variation thereof and tell people to live with it or do it otherwise.

I don't think I have the engine set up in a way to handle the true NFL rules since we don't handle rosters the same way. I'll add it to the list to look at in the future, but it won't be in for PF21.