Ah, Allan, multiple email updates by you (the Admin) are an entirely different kettle of fish!
You can do what you want to do with Directory Import. The email addresses you see on the email addresses tab of a mailbox are broken out into two pseudo import/export fields: 'E-mail Addresses' and 'Secondary-Proxy-Addresses'. The two fields together make up the contents of the email addresses tab of the mailbox. By definition, because you can't have more than one primary proxy of the same address type, the 'E-mail Addresses' field (not to be confused with the 'E-mail Address' field, which belongs exclusively to a Custom Recipient) will contain only one address of each type, the bolded primary. (If you have more than one type of address defined, all the primaries will be in this field, separated by % by default) All other (non-bolded secondaries) will be in the 'Secondary-Proxy-Addresses' field, all separated by % (by default). If you have more than one secondary proxy for a type of email (say SMTP) then they'll all be there, and breaking them out can be a test of your Excel or scripting skills.
If the username bit of your email addresses isn't changing (just the domain bit), you could export the 'E-mail Addresses' field for your users, extract just the SMTP addresses from this (by Excel's Text to Columns command, or via a formula that searches for SMTP: and extracts the address, or via a perl script or similar), do a mass search and replace on your domain.com bit, and rename the column 'Secondary-Proxy-Addresses'.
When you import, make sure that the option for Multivalued Properties is set to Append (the default, I think), not Overwrite, otherwise you'll blat all your other secondary proxies! You can test the effect of your import by editing your import file down to just a single mailbox line to test the effects before you repeat for all the mailboxes.