Dude, stop and think. There's nothing magic about the GOG version, the import option looks for files that match the GOG version and downloads anything it can't find a match for. Most likely it will think the Steam files are a GOG version that is slightly corrupted.

I don't have TW3 but I'm going to try this with a different game when I get home.

First, is your internet slow usually?
If not, then be sure to download using the GOG Downloader, not Galaxy. Galaxy can be very slow at the moment.

Second, if your internet is slow but your buddy's is not, then I would recommend taking a laptop over (if you have one) and download your GOG files from the GOG downloader. Then copy those files onto your desktop if you play on a desktop. If you have no laptop, bring a portable hard drive, flash drive, or something to store it on and use one of his computers.

They're not corrupted, damaged etc. They're changed so it can work on Steam and on Steam only.

GOG won't fix it!

To answer your specific question, no it is not possible to do what you're asking. To try to solve the underlying problem however which is that you want to get the game as soon as possible there are 3 different ways you can download and install your games here on GOG at the moment. If one of them is acting slow for you, you can try one of the other ways and it might be much faster. The 3 ways are:

1) GOG Galaxy - which is probably what most people new to the site and many others are using currently because it is prominently displayed on the front page and elsewhere and people are encouraged to try the beta. The beta is incomplete and some people report having problems with download speeds with it. This could be due to bugs in the software that are not ironed out yet, or due to slow servers or other problems specific to a particular country or region.

2) GOG Downloader - which is the main way people downloaded their games before Galaxy and probably still the main way most customers that have been here for a long time still use. GOG Downloader is fairly stable and has been around forever. It also uses different backend servers from Galaxy so performance problems on one service do not affect the other.

3) Directly downloading the game installer from the website library and installing it manually. I believe this gets its files from the same place that GOG Downloader does so they should in theory have the same performance characteristics. For large game files many people find the GOG Downloader to be more reliable than using a web browser to do their downloading however.

If none of these methods perform well enough, you can contact GOG support and they may be able to provide further assistance. Ultimately they want to give you the best experience they can and if you're not having it then they want to help.

Hope this helps and welcome to GOG.com.

Yeah, but Galaxy doesn't see that they're Steam files. Galaxy only sees that "something's wrong" with the files. It's no as if Galaxy opens the files and reads their source code. It probably just checks some hashes and redownloads files were the hash sum is wrong.

And reads 'Holy s$#@! It's not the GOG version! Abandon the ship! GOG games first, no place for pirated and Steam versions in the boats!' :-)

Wow thanks all! I think I will try to copy my friends first, if it won't work, I will bring my harddrive to my friend and download the game from his pc, he has fast download speed connection, thanks for all your suggestion! I appreciate it ! XD

Let's play the witcher 3~

No. It reads "Holy shit, what happened to these files? They're horribly corrupted!" We're not talking about adding the files of a game that you don't own to Galaxy... Galaxy knows if you have The Witcher 3 on GOG ;)

In this situation there's no difference between changed and corrupted. The point is that the files that aren't changed will be picked up and the ones that are changed will be replaced. No offence, but you don't seem to know much about how this sort of thing works. I'm not guaranteeing it'll work, but I have reason to believe there's a good chance of it working. At least enough to make it worth trying.

As I said earlier, I'll try this myself at home with another game.

Offence taken. Will you be able to admit that I was right/wrong after all? Please do.

Even if that copy somehow worked, wouldn't the OP end up with his friend's key?
Also, would the game even link to his GOG account?

EDIT: ouch forum hiccup

Hey, you're the one categorically stating that it won't work without demonstrating an understanding of the process behind file verification. Even if it doesn't work it won't make you right.

It's up to you to prove it works, not me.

No, because if it worked, it would be because Galaxy has overwritten the things that are different from what it expected to find.

Think about it this way, it would effectively be installing the game from Galaxy and just not bothering to download the bits that are the same between the GOG and Steam version and overwriting anything that is different.
I'm not categorically stating that it won't work, if anything the burden of proof could be said to lie with you. I'll try it with Terraria though since it's not had an update for a while I know the GOG and Steam versions are the same apart from the Steam version not being DRM-free.