No. You can add any map, but you have to rename it, so it replaces another one. For example:
The level you downloaded is called "InsideYourNose.j2l", and the level you want to replace is called "share7.j2l". You must rename that level to "share7.bak", or something like that, or directly delete it, and call the level you downloaded "share7.j2l".
I hope you understood...