Note that this method may not work for all games, and I have not tried it for this specific game, but it has been successful for me in the past.
- Install the game through Steam
- When it starts downloading, pause it and close the Steam client completely
- Copy the files to folder Steam created, overwriting as necessary
- Start steam
Now there could be a few situations:
- Steam does not recognize any of the files, and will continue to download the full game
- Steam does recognize some or all of the files, and might download only a few additional files
Either way, after it has finished, make sure to verify the integrity of the files by following the steps found at this link:
- Restart your computer and launch Steam
- From the Library section, right-click on the game and select Properties from the menu.
- Select the Local files tab and click the Verify integrity of game files... button.
- Steam will verify the game's files - this process may take several minutes.
Keep in mind that even though the game might look similar, there could be a lot of structural changes between a testing version and the release version, or even between a normal version and a Steam version. As such, this method might not work at all because there have been too many integral changes.