You CAN make your background clips appear in the FCP X Generators background area, though it’s probably not an ideal way of working.
By taking a clip into Motion and saving it, when you save it tick the box that says publish to fcp generator or words to that effect.
Set the category to Backgrounds in the pop-up menu that appears. They will now appear back in fcp in the backgrounds category.
As you already tried though, it’s probably best to import them into an event as required. Make sure it doesn’t copy the media on import. Then you can just leave all your stock video / gfx footage all in one place, and it will just create an alias.
By importing them into fcp as a generator via motion, it’s going to end up creating a whole lot of extra data in your…
user name / movies / motion templates / generator directory.
Hope that helps.
Cheers,
Brendan