Nope, you can only use JCS. However, if the source code was released, then there could be a few fan-made level editors around, like in some other games. And about the idea of a 1-2-3 level maker, it could be interesting, but most of the levels would be nearly the same as each other when made by it.

Not only that, but if the source code were released, we could have super-nifty cool stuff like Mods, (not the music kind, the modification kind) like ST said. And Matrix, sorry, you can't - there is no source code.