Magnet regions have a single transition timeline that is entered to get into it, which sounds like what you are asking for. It’s not quite a loop region - it just acts like one because if you leave it, you will have to re-enter it (which happens via the transition timeline if there is one).
If you want a separate transition timeline for looping, you would have to add a loop region with a transition timeline inside the magnet region. Let me know if there’s something I’m missing with what you are asking for.