I dont think you can bulk edit existing users with the type of edit you want to do. You can do about every other bulk edit in the world, BUT.. that one. If you were changing Domains, you could change the Exchange Receipent Policy in the "Messenging" server. For instance, every time you created a new users, you can have the domain be *@company.com. Or another recipent policy update @company.com to @companyb.com and it would add this to their smtp addresses.
The only way I think you can do this, is to have some create a SQL script to go in and change the Alias table on of the unityDB. You can actually open up the unityDB table and find the ALIAS section. Update one manaul and see if works. If it does work, if you are a SQL guru (I'm not ugg.) have them create a script for what you need.
hope this helps.
