You can't add a property to a type unless you are an administrator of that type.
It looks like interior artwork is attributed through the issue's Contents property, which links to the comic_story_printing CVT, which links to comic_book_story, which already has properties for penciler and other artists. This was probably done because a single issue might have multiple stories illustrated by multiple artists.
So, it is already possible to add the penciler of interior artwork but you must do that through comic book story instead of issue. It would be nice though if you also linked story to issue through printing by attributing all those properties too.