Change reply-to address for exchange account

I'd like to use an exchange account for my email but don't' want to move the domain over to 365. I can see how to do this with an IMAP account when using Outlook 2007 but not an exchange account, is this possible?

Example: Currently my mail address is sam@myco.com and has an IMAP mailbox with a hosting company.

My 365 domain is myco.onmicrosoft.com. I'd like to forward all mail to sam@myco.onmicrosoft.com and use Outlook 2007 and my iPad to manage mail from that box. When I reply to mail though, I'd like recipients to see that it has come from sam@myco.com. not sam@myco.onmicrosoft.com

Thanks

Sam