MidnightDaystar, what exactly do you mean by "copy multiple tiles"?
You can select several tiles in the tileset and insert them into the room by a single click in the current KSLC version.
However, what I suppose you meant was to copy several rooms from one level to another, right? That is unfortunately not possible at the moment, and generally such a feature is very difficult to design, so I don't think it'll be available within any foreseeable future.
Note, it's difficult to design, not implement. There's just too many things to decide what to do about when copying rooms; the actual implementation then is rather trivial.
A simpler approach that might be implemented quite soon is importing a (complete) level into another level. This is way easier, because most of the world.ini settings stay intact. Still there are issues, such as tileset collisions, custom objects etc. that need to be solved.
Anyway, nothing's gonna happen in the nearest future (month or so), since i'm working on different projects now and simply don't have the time needed for KSLC.
Still, any programmers who'd want to join are welcome, KSLC is, after all, open source.