Steam support for Rift without re-downloading?

Solution 1:

Steam-enabled games can only be played by downloading them through Steam; you can't have them downloaded from another location. Unfortunately you have to re-download it because Steam is using its own files for games.

Solution 2:

Let Steam download 1-2% of the game, then close Steam. Now browse to the SteamApps\downloading folder under Steam's install location, and place your existing Rift files inside the gameid folder (39120). Open Steam and it should start performing a check of the files' integrity, with chances being that only the game executable and steamapi.dll would need a refresh.