I tried it on a mail merge document of my own and used the FILL-IN field, which runs when opened if a template and will run again when you run your mail merge. Use prompts for the answers you require. If you have a lot of mail merge recipients this method I think will become confusing. I tried using Content Control in the Developer Tab, but once I ran the merge, it would only allow the same result as the first record and I could not restrict the editing either (probably not enough knowledge on my part me thinks - I shall be improving my knowledge in this area).

Have a look at Graham Mayor's website www.gmayor.com, you might find what you are looking for there.

Might be an idea to upload an example of your mailmerge document and the fields for the mail merge and the prompt required for the other fields that need to be entered manually.