In iMovie how directly import videos from non-"System Photo Libraries"

iMovie allows you to directly access/import videos stored in the Photo Library that you have designated as the "System Photo Library". I have a lot of performance videos that I've stored in a separate Photo Library - not my "System Photo Library". I've done this because there are many long videos and I don't want them cluttering up my "iCloud Photo Library" stream.

The issue is: how to import those videos directly in iMovie without first doing an export to the Finder from Apple Photos and then bringing them into iMovie from there. I've tried dragging them from Apple Photos into iMovie (doesn't work) and navigating to the library using the iMovie "Import Media" function (the libraries are grayed out).

Am I overlooking a direct method?