Mathew found an undocumented feature of the LibraryImport command. You can import datawindows into a PBD. The documentation implies that you can only import into a PBL file.

However if you want to create and re-use dynamic datawindow in your application they have to be in your library search path. This is great when you're dynamically creating datawindows from a database (using .Create) which have Dropdown Datawindows on them. You need to import the Dropdown Datawindows dataobject definitions into the library otherwise you can't see them on the main datawindow.