Merging 3 WPMU sites into one

I have 3 different WPMU installs for 1 website on 3 domains. I've wanted my users to be able to select which domain to keep their blog under.

Now I want to look at options to use one install to do this. I think I've find a good plugin for it. The multi-site manager doesn't do it since it'll sign up username.site1.com to username.site2.com and username.sige3.com. I think an other plugin I found does what I want it to do, though. If anyone knows of a plugin to do this, please let me know.

Anyway...

If I'm able to do it, I want to merge the 3 sites.

Is there a tool to do this? How would you do it? I should be able to move database posts and change the blog-ID, right?

Thanks.