question about usage

This may not be an appropriate place for this question...

I'm trying to implement an invoice-type form using XForms.
I have the examples of inserting and deleting from repeating
groups of elements.

What do I do with an xml file that does not have any
elements of some optional type, where I then want
the user to be able to insert some of those elements?

Do I somehow load up a "dummy" xml document that contains
spaceholders for all of the optional elements and then
merge that with my true data?

Looking for suggestions,
Chris