If the game has the last official patch, it will be compatible with the launcher. In anycase, all you need to do is add the RoR launcher to the game folder and run it each time to play. It updates the client to the current RoR patch, so steam will not be required for the game.
As Lilim posted abover, you can then add the ror launcher to steam client as a "non=steam game" to your library.
If it doesn't work, just download the version throught the client download topic.