You'll need to either create a side plugin for ProjectKorra that does this, or edit the default collisions by editing ProjectKorra's source code. Both of these will require you to know a little Java, however.
If you want to edit the source code, the source code is available on GitHub ready for compiling. The file you're looking for is CollisionManager.java (I believe - going from memory).
Hope that helps
Edit: Moved to development support