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