Can I use Office365 e-mail to a subdomain while keeping my hosted IMAP in the primary domain - requires multiple MX records?

One of our company entities is currently using the hosted IMAP e-mailed provided for free as a part of their website hosting account.  I really want to move them to Office365, but the price difference - from "free" to $$$ - is making it a hard sell.  The companies has a group of employees - 30-40 - that really can get by with the IMAP hosting.  Another smaller group of 10 or so employees would really benefit from the features of Office365 hosted e-mail.  My thought is keep email to @mydomain.com pointing to the IMAP server via its MX record.  Setup Office365 to a subdomain @admin.mydomain.com and have its own MX record setup.  This seems possible, but I don't exactly know how Office365 setup will go in a subdomain.  Also, If I setup Office365 in this subdomain environment, would it be possible to move it to @mydomain.com if we ever reached a point where we were OK with paying for O365 subscriptions for all of the users?