You can just copy your game folder over to steamapps / common/ folder. Steam won't recognize it at first but it will eventually recognize the game as having been installed. (takes me a couple days/reboots)

Alternatively, you can start the download on steam, pause it, then overwrite the game folder with your own.

If you still have the original install, Steam actually has a back up option that's very convenient. You can even resize the files and it'll give you multiple files in the size of your choice. You can just use the back up files to restore the installation.