First, please forgive me for not having seen your post but I must have missed the automated notification I have set for myself.

To answer your question, every UBL document is extended with user-defined extensions in the same manner, that being through the UBL Extension point. On our home page there is a link to http://docs.oasis-open.org/ubl/guidelines/UBL2-Customization1.0cs01.pdf that describes how to customize UBL for your own needs. Essentially, you use your own namespace to define the apex element of a tree of your own extensions (that can reuse UBL components), and you put that tree under the extension point at the start of every UBL document.

Note that questions such as these are best asked on the UBL Dev developer mail list. I've outlined how to subscribe at the top of the http://ubl.xml.org/forum page.