didn't tested for now if enemy skills could be used, but I suppose these are the ones with the big (E) at the end of the name.
I've currently omitted some of the skills from the dropdown that don't have a localized name as the list was getting waaay too long, so if you want to see *all* skills check SkillData.txt in the game Stats\Generated\Data folder.
If you want to add a custom skill, ie. one that is added by a mod, or is not displayed in the list because its unnamed, just click "Add skill" when the "--- Select a skill to add ---" item is selected, and double-click on the newly added skill name in the skill list to rename it.
What'll be your next objective?
Inventory editing probably.
Also an editor plugin is currently under development that should (hopefully) help with modding and merging the stats .txt files.