Convert existing site to network - WordPress Answers

Convert existing site to network - WordPress Answers: "Setup

First, install WordPress normally. You'll need a fresh database and a fresh download of WordPress 3.0.3.

Next, add define('WP_ALLOW_MULTISITE', true); above the line that says, /* That's all, stop editing! Happy blogging. */ in wp-config.php.

Now, go back to your site and go to Administration » Tools » Network to activate the network installation. You'll be given some additional information for wp-config.php ... copy this code in between where you wrote define('WP_ALLOW_MULTISITE', true); and where it says /* That's all, stop editing! Happy blogging. */. Don't change anything else in the config file."

'via Blog this'