I have not found the need to save mailmerge main document files as
templates.  If you want to do that, you will need to have an autonew macro
in the template that opens (attaches) the data source to the new document
that is being created from the template.

ActiveDocument.MailMerge.OpenDataSource Name:="[drive:\path\filename]"

My experience has been that the data file remains attached. Recently I moved
my data files and it was a real pain. I ended up detatching the data file
from the template but writing a macro that would attach it when a new
document was created. (My macro is more flexible than Windows and easily
finds the data file even if I move things around because the data file is
kept in a subfolder of my workgroup templates folder. The macro finds the
workgroup templates folder setting and goes from there.) Doesn't answer your
question but it might help you work around your problem.

I use templates for mailmerge main documents because I almost always using
the merge with one record to create a custom document. I simply display the
data, lock the fields, and save the new document with the individual data
frozen in place. (This is also done with a macro that changes the document
to an ordinary Word document from a mailmerge document. The AutoNew macro
also calls up the select a record dialog.)