Thanks for sharing your scenario.  Note that for general discussions you should probably post your question to UBL Dev per the suggestion at the top of http://ubl.xml.org/forum 

The two steps you need are (1) - create a document schema for the definition of the document element, and (2) - create a private common pair of aggregate and basic schemas and put what would normally be added to the committee common library into these two private schemas.

This follows the CCTS structure implemented in UBL by having the one schema for the top-most elements, and the common library of components.  But you can't just place your new constructs in the existing common library because the namespace is reserved for the committee, so you put them into a parallel private common library for the time being.

I say "for the time being" because I also suggest you join OASIS and submit your new documents and private common components to the UBL committee for consideration in future versions of UBL.  They will already be in the correct hierarchy, it is just a matter of convincing the committee to move them into the committee's common library.

I suspect it is too late for UBL 2.1, but there is a meeting starting next Tuesday in Copenhagen if you want to see what is going into the next revision of the specification.

The common library is structured this way specifically so that people can reuse it for their own documents in addition to the UBL documents that are using it.

Good luck in your project!

. . . . . . . . . . . . Ken