My second CD seems to be broken (insalation crashes, can't copy files on disc), is there any way to add the game to GoG without installing it. Manual states that I need a game shortcut to do this.

There was, my problem was not getting a code, but adding a retail game on GoG as manual told me to use the game's shortcut to do this, which I coudln't do without installing the game in the first place. I've now found the option, which was hidden under the "plus" button, so my problem is solved.