Also curious about that.

Yes, if your app is to use the pencil you can develop for it. Your Swift Playgrounds app project must be built with and run on Swift Playgrounds 4.0.2 on iPadOS 15.4 or Xcode 13.3 on macOS 12.3. If it runs on iPadOS, it must be optimized to display properly on all models of iPad Pro.

No, 3rd party asset is an asset made by someone other than you. You can't use somebody elses work on your playground. But you can use anything you created.