Bulk Edit can help if you're going to do this using a pattern (i.e. adding a "54" to the left of every extension or stripping off the leading two digits etc...) but it's not capable of reading in a CSV file or the like to change groups of extensions based on file input or the like.
However, the Cisco Unity Bulk Import tool (CUBI) can read in a CSV file and update extensions (and transfer strings if necessary) for existing subscribers - you'll find the tool in the Tools Depot (assuming you're running a recent version of Unity - you didn't indicate what version you're at) - check out the help on the tool for details on how to setup the CSV file to accomplish this.
