not sure if theres a way. but here's what i did: u download the game again and when it asks you to choose a file location, choose the location where the already downloaded version is in. the files have the same name so it will ask you if you want to replace the file. then click yes.
I came here looking for a way to add this game on my steam without having to pay for it AGAIN. According to your comment section, it should be possible. So why isn't it available? I hope this will be available soon! Thank you <3
