I'm not sure how to do that....my "merged" document shows all the envelopes
with the addresses, but no return address. I picked up the addresses from a
mailing list that was in an Excel format.
At the point where you edit the merge document (not the merged document) to
add your address fields you can add a return address to the envelope, which
at that point is simply a document like any other. Then re-merge the data
from your Excel mailing list. - http://www.gmayor.com/changing_envelope_layout.htm