Thanks for posting in Adobe forums. Unfortunately, it is not possible to add AfterEffects Project file in Libraries & use it as an asset. But you can add the project file in Creative Cloud Files & access them using browser or sync to the Creative Cloud Files folder in the machine.
All the product folks at Adobe listen.
1. You say that it is possible to after effects add project files in CC Files
2. You say AEPs not possible Libraries (which simply accesses files from CC, if you inspect closely)
These are contradicting statements. On to you just make the statement but do not reason why not. Start listening to your users, you are already losing lest not you become fax machines.
A cousin of mine turned into a fax machine. It's no joke.
I agree this would be most useful. Not sure why it's not possible.