Your question lacks clarity. Your post title refers to mailmerge, but nothing in the post itself indicates how your 'text templates' (whatever they are) might relate to a mailmerge.

Please clarify what your 'text templates' are and how you would propose to use them.

A Word mailmerge main document can use a text file as the mailmerge data source - provided the text file is suitably structured (e.g. one line per record, with a field name for each item on the row, and fields separated by tabs or commas).

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]