Just intall WP 3.0.1 on a test server and enable Network. Here's a tutorial for that.
Then create new sites. If you have selected subdomains, it will look like this:
Go you your existing sites, and export pages and psots (and any users).
Now, copy your WP themes from you sites, and paste them to you WP 3.0.1. themes folder.
Enable the themes in Super Admin menu.
Go backend to one of your sub-sites and import the data.
Then enable the theme.
This is a very rough guide, but hopefully it will guide you in the right direction.