I think that your description of the situation may not be correct.

Are you sure that it is formfields into which the data is be merged from the
external database?  If they are formfields, that that could not be being
done by using mailmerge as mailmerge can only be used to populate
mergefields.  Of course there are ways other than mailmerge to transfer data
from a database into a document, but they would involved the use of VBA, and
if what is actually being done is that the .Result of some of the formfields
is being set by using VBA, then it would also be possible to use VBA to
unlink those formfields which would turn the data that is inserted into them
into ordinary text and then when the document is protected for forms, as it
will need to be to allow other people to populate the remaining formfields,
they will not be able to access or change the data that was merged from the
external data source.

So the question is, how is the information from the external database being
"merged" into the document.

Sorry answer your question with a question, but if you can answer that, we
can give you more information on what you need to be doing to be able to use
the document as you want it to be used.