Is a gradual migration from a workgroup to a domain possible?

Yes. You can access files if you know the username / password of the server. So the workstations can temporarily stay outside the domain until you migrate them.

I've actually done this with ~5000 computers, but I had power tools (Altiris Deployment Solution). You should have no problem accessing shares.

The thing that your users might complain about, however, is their profiles. You might need to use some kind of profile migration tool (Windows has one built in) to move their local account profiles to their domain account profiles. They'll need their documents, their application settings, their outlook nickname file (our users were very attached to their outlook nickname file), and, depending on your environment, perhaps even their wallpaper.