I downloaded Rift for my PC a few months ago. But seeing that it has been released on Steam now, I want to play it with Steam Support.
Can I do this without having to re-download the game?
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Sign up to join this communitySteam-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.
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.
What kinds of Steam support are you looking for? If you want to avoid the re-download (required since the files are different) you can make it a non-Steam game shortcut and launch with Steam to have the Shift-Tab menu and all manner of features — just not the integrated kind. If you want those features then you will indeed have to re-download it.