For an existing release you can look at e.g. KeymapProfileFor60 to see the specification.

If you are developing a module for NetBeans development builds, you can just look at this file in the section Shortcuts/. This will show all global keybindings being used by modules in the standard IDE as well as experimental update center in http://hg.netbeans.org/main/ and http://hg.netbeans.org/main/contrib/ as of a few hours ago.

Editor-specific keybindings are listed in Editors///Keybindings/ folders, which is unfortunately harder to browse through.

Be conservative about adding new keybindings; they are a precious resource. Be careful with bindings using Alt, as these often clash with mnemonics, Linux window manager shortcuts, etc. If at all possible, use a multistroke binding: for example, Shortcuts/D-J R C.shadow binds the 3-stroke sequence Ctrl-J R C.