Install your keyboard by cloning it into Shelter or by installing it through an App Store that is installed / cloned into Shelter.
After that, you can activate your keyboard in Settings > System > Languages and input > On-screen keyboard > Work.
The reason why Android enforces a separation on the keyboard is simple: Third party keyboards also grab data. If you choose to use a third-party keyboard, it should not be able to combine your data. Shelters goal is the strict split between your data to provide isolation and thus more privacy.
EDIT: This will not include data sharing which you're apparently after. It wasn't clear to me before you replied to me. If it's just about getting the "same keyboard" (in "same APK" without data sharing), this is the right approach.
An alternate solution for your problem with KeePass
I had the same problem, here is how I solved it:
I've setup Syncthing on all my profiles and my PC (should work with Shelter as well) and let all profiles sync a KeePass database. Use the Syncthing-Fork from F-Droid in particular, it offers better diagnostic capabilities. Alternatively, use another Cloud provider if privacy is not a concern.
I use KeePassDX from F-Droid on all my profiles. It allows to open a local KeyPass database and supports quick unlock with a PIN. It works well between profiles.
Good part about this solution is that your KeePass database is synced between two devices. It works with multiple Android user profiles and should also work with Shelter.
If this doesn't suit you, there is only the old copy paste between your KeePass app and the isolated Shelter app, because the clipboard is shared between both profiles.