Could you add a method to your addin that exports sharable addin
macros into a VBA module and inserts that module into the current
workbook?

You may have to modify your add-in macros so they can be run as both
add-in macros and workbook macros, that is, by having Workbook and
Worksheet parameters to all the Subs and Functions, but it's not a big
deal.

Check out the Subs and Functions for the VBProject object to get
started.