Create the mailmerge main document as a template and have the users select
File>New to create a new document from that template.

Signature
Please post any further questions or followup to the newsgroups for the
benefit of others who may be interested. Unsolicited questions forwarded
directly to me will only be answered on a paid consulting basis.
Hope this helps
Doug Robbins - Word MVP
> Do you know any workaround to allow merging into a read
> only form? Data source is not read-only (txt file), but
> end users continue to save modified merged file over
> original.