I saw a discussion on the GP Newsgroup about using an Excel spreadsheet as the source file for the Professional Service Tool Account Modifier/Combiner. Mariano had promised a post and in the holiday fun, I missed it.
Mariano has more in his post.
Excel as a Source for Account Modifier/Combiner
Excel & OfficeDevelopment & Integration
Archived post from 2008. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.