A number of institutions, including the Bentley Historical Library at
the University of Michigan, have used macros in Microsoft Word to
generate EAD. See  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. 
Mark A. Matienzo
Digital Archivist, Manuscripts and Archives
Yale University Library