In order to convert your retail copy of Borderlands to the Steam version, please follow the steps outlined below:
You’ll need to find your game disc and you’ll need to create a free Steam account. http://store.steampowered.com/about/
Navigate to your game “Library” menu, then select “TOOLS,” and locate the Borderlands Granting Tool. Install this tool.
Installing and running the Borderlands Granting Tool will guide you through the process of getting you a Steam version of Borderlands. NOTE – You will need your Original Game Disc to prove ownership.
After the tool completes, you will be able to install Borderlands from your Steam Library.
What about my DLC or Game of the Year version?
If you purchased any DLC through either the official Gearbox or 2K Stores, or purchased a Borderlands PC Game of the Year Edition outside of Steam, you will need to find your original activation key and redeem it within Steam AFTER unlocking Borderlands on your Steam account. For instructions on how to do that, please refer to the Steam directions on activating a key:
If you have lost your officially purchased DLC Key, you’ll need to visit the Gearbox Support site and follow the steps for recovering a lost key.
If you are unable to recover your Game of the Year (GOTY) code voucher, please submit a support request.