Daniel,

A number of institutions, including the Bentley Historical Library at
the University of Michigan, have used macros in Microsoft Word to
generate EAD. See [0] for example.

Arguably macros are difficult to maintain and require overly precise
markup in Word of things like headings. I can't begin to estimate how
long it would take to modify and maintain the macros. You may want to
consider using them for an "initial push", and then create/maintain
EAD finding aids using some other set of tools after you convert your
existing finding aids.

I would also urge you to consider posting your question to the EAD
Listserv as well. [1]

[0] http://bentley.umich.edu/EAD/bhlfiles.php
[1] http://listserv.loc.gov/cgi-bin/wa?SUBED1=ead&A=1

Mark A. Matienzo
Digital Archivist, Manuscripts and Archives
Yale University Library