Can you use a formula field in Marketo to change a sales rep's email domain?

Hello,

      I am looking for a way to change the email domain for our sales rep dynamically so I can send Marketo emails from an assigned sales rep with a different domain.

We have a unique situation where we cannot add CNAME records to our existing branded domain, companyname.com. With Marketo, we have a secondary domain, companynamesolutions.com. The issue I am having is that all our assigned sales reps have an email address of companyname.com as their sales owner email in Salesforce.

What I am looking to do is to somehow take their email address, strip off the first domain and append the second in an automated way:

example:

Jason@companyname.com > Jason@companynamesolutions.com

We already have an email alias set up so sales can receive emails from this new domain, but not sure how to dynamically it and place the new value in a second field that can be used with tokens. I thought about doing this with a smart campaign and manually matching the values, but we have over a hundred reps and managing this over time will be a pain as reps come and go.

Any ideas?