Yes. The files are just files.
Galaxy *can* import folders, but I'm not sure how well... it fluffed moving one of the other titles and started re-downloading.
I did just download W3, and then manually ran the installer .exe, and Galaxy detected the correct installation as it happened.
Set Galaxy up to store it's files into <Somewhere>/!Downloads/, then pop the installers into a sub folder the_witcher_3_wild_hunt/ and you should be ok.
Worst case is it gets confuddled and you have to either re-download, not use Galaxy or have another go at persuading it to play nicely...