Migrating single sites to MU

I’m in the process of migrating several of my single sites into a MU installation and using domain mapping. One of the single sites has 8 email accounts and these users are all using IMAP and leaving a lot of their emails on the server. Both the single site and the MU site are at Hostgator but on different servers/IP numbers. Is it possible to leave all these email accounts configured as they are on server A and move only the site content to the MU site on server B? If so would this be done with DNS configuration and how would I configure it?