Is there a way to convert an exsisting AD mail user to a user mailbox in exchange 2007? When I go to create a new mailbox for existing users none of these accounts show up.Thanks,

Hello, You need to remove the exchange attributes from mail enabled users with Disable-MailUser command then you will be able to see existing users while creating new mailbox.

That worked thanks!

Great.. Check below post also as an advance reference.. http://forums.microsoft.com/TechNet/ShowPost.aspx?PostID=1925161&SiteID=17