I love the fact that GOG is working on an integrated library. This change might finally help me to move over all my business to their client. However, my steam library isn't in the default path but rather installed on my secondary hard disk. Is there any way to add all my games from that library?

It shouldn't matter where you steam games are installed. Simply adding the Steam integration should import all of your Steam games. I actually have Steam games installed on multiple drives and have had no problem with it. In fact, if you tell GoG Galaxy to install a Steam game, it will pop up the normal Steam installation dialog, which means that you can select which Steam library to install to.