Dragon Age games on Steam accounts have a product key that you can use to register them with on your BSN account. Log into the BSN at http://social.bioware.com/user_bw_games_register.php and enter the product code. If you have never created an Origin account before, you can create one at that page.

To find your code in Steam, go to the game's page in your library - there should be a "CD Key" link in the right sidebar. There are separate codes for any purchased DLC so get those as well. The DLC is registered on the "Redeem Promo Code" page at http://social.bioware.com/redeem_code.php.