You can already add cast / crew data as embedded metadata which should be picked up by Plex – though, Plex will treat this as a pure local entity without any links to the Discover Credits feature.
This also currently doesn’t allow adding pictures, bios etc… just names.
There’s some more related feature suggestions which already address what you’re asking for, e.g.
I suggest you comment/vote in those threads in order to help us avoid distracting or cannibalizing votes. Unless I’m missing some unique aspect of your suggestion, I’ll close this thread as a duplicate.
