@K3fka: When I was running the leadership, things like that were completely open source (like, nobody needed to claim anything, but could make use to whatever possible, that later would be reorganized and ported to the Base). I guess it's still working the same way, but if I'm wrong, MrDeppel will let everybody know later.
Just the thing about external resources is that you may ZIP everything you've used together with your level. Anyway, I think there's no need to worry about slots and the like.
Relevant reminder edit: Custom block behaviors should be inserted on Page 3. Actual tiles that you may use in the level may be used in any other page, but with the custom behavior.