Apparently, there is no easy way (this or this do NOT work), there use to be a tool to do this (Moveuser.exe), but it stopped working with Windows Vista.
The way to do it now is to write a VBScript that uses WMI, happily, I have found someone that has already done it here.
It basically seems to be using the ChangeOwner Method of the Win32_UserProfile Class, one problem I have found so far is that if the profile to be “moved” in to the domain is heavy (many Gbytes of weight) the ChangeOwner method can take a really long time to do its job. A possible solution then is to move the files outside of the profile dir, run the migration process, and copy the files back (you might need to change the permission information of those files to be able to copy them back)
