My Internet connection is R.E.A.L.L.Y slow (3,5Mbit max), so I'm generaly importing already downloaded game on my own account somewhere else and play.
So far, it worked with every game..

... until I owned S.T.A.L.K.E.R. Shadow of Chernobyl. I'm unable to import this game.
Downloaded 2 times, checked files integrity.. everytime I try to import it it says:
"Unable to find such game".

However, for comparison, S.T.A.L.K.E.R. Call of Pripyat import works correctly.

Seems to be GOG client bug???

I can offer no help with that. But at least you can play the game without Galaxy... not as convenient, but since there are no achievements and stuff it should not detriment from the experience.

Yes, but.. what about registry keys, installation paths and so?
The only way when it works is to just download full game (over 6 Gigs) - it's something like ~5hrs overloaded ISP connection and increased pings. Even with QoS enabled they're high.

EDIT: It worked.
1. Download game somewhere else
2. Backup all files
3. Copy all files from backup to the same system path and folder names
4. Try to install game (instead of import!)
5. GOG will find game and install it