Migration from single installation to multiblogging

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:

sub1.domain.com
sub2.domain.com
sub3.domain.com

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.

Thank you spstieng!

But can I use different domains instead of subdomains?
I already have these six sites on-line and I don’t want to change their domains but just to manage them from a single dashboard setting different themes, plugins and contents.

Yes.

You can use the Domain mapping Plugin found here:
https://www.ads-software.com/extend/plugins/wordpress-mu-domain-mapping/

With this, you can map yourdomain.com to sub.domain.com. Here is a good tutorial on how you can use Domain Mapping:
https://ottopress.com/2010/wordpress-3-0-multisite-domain-mapping-tutorial/

Here is a step-by-step tutorial on how you can enable multisite. Its written by Andrea (a die hard WP developer).
https://wpebooks.com/2010/09/how-to-enable-multisite-in-wordpress/