The Steam version of a game is actually built differently in order to rely on Steamworks DRM instead of CD checks or other varieties of copy protection. From just a binary standpoint, there's typically no way to convert the "Retail" (ie, non-Steam) edition of a game to the Steam variant.
The major exceptions to this are games that either only use Steamworks (ie, Civilization 5) and games that Steam will accept CD keys from, which is limited to games on this list.
Happily, Borderlands is one of the games on that list. If the game will give you the CD key, or your friend has the key with him, you could log into a Steam account and redeem it, thereby giving him access to the Steam edition.
However, then Steam is probably going to want to download the game from their servers, which may take a while. You could try to figure out some way to copy the data files from the retail version into the locations Steam would have downloaded them to in order to fake Steam out. This doesn't always work for every game though.
MBraedley points out in the comments that you can also use the Steam version installed on the other machine to speed up this process, either by backing up and restoring the game via the Steam interface, or as WernerCD points out, by copying the files directly. This is likely to be a far more successful method of saving the Steam download.
Also, we've kind of entered the phase of the problem where the "alternative" solutions start to make more sense. (I am, of course, obviously referring to making your friend go get his CD.)