Hi Fraktal,
You can indeed modify existing levels! I just put up a tutorial for this here:
https://docs.larian.game/Modding:_The_main_campaign. The tutorial was written for modding Story levels, but the workflow is the same for modding GM levels (just make sure you target/load data from "GM" instead of "Story" when creating your project).
For modding levels created by other modders, you may have to take a look at this recent forum post by fireundubh:
http://larian.com/forums/ubbthreads.php?ubb=showflat&Number=620801#Post620801.