Even if you were to go down the path you're suggesting - which isn't practical - you'd still have the problem of people editing the document. It's not much good having a set of mergefields at the front that someone might mess with and, even if they don't, they could still make a mess of everything further down.

Solution: Finalise your mailmerge main document's content & structure, then protect it against further edits.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]