I think I understand what you are trying to do...
You want the users current local profiles to be rolled over to their new domain roaming profiles, right? It is possible, but I am not sure what you have to do, becuase I never tried to do it myself. A company called Aeilta has a domain migration software package that does migrate the user's old domain profile to the new domain profile, and from that I know it must be possible.
My guess is you need to change the permissions of the profile and then copy it out to your file server that is holding the profiles. I would try messing around with the User Profiles in the System Properties (right-click My Computer, select Properties and click on the User Profiles tab). Within there, click on the "Copy To..." button. There is a "Permitted to use..." option, where you would change this to the user's domain account, and then there is the "Copy To...", where you would copy the profile to the server. In the user's account properties, point the Profile path to the user's profile (that you just copied) out on the server.
Hopefully you don't have a large number of users. You would probably need to script this if you do. Try testing on yourself, or a test account.