I purchased the game before, twice. I lost my first physical copy, and I was wondering if it would be possible to use the CD Key from my NWN2 Manual.

You definitely can, just go into your game install folder, find nwncdkey.ini and replace the keys there. (key1 is the original game)
(http://www.gog.com/forum/neverwinter_nights_series/nwn2_how_to_play_multiplayer_and_change_your_cdkey a better guide)

But it depends on what version of the game. For multiplayer, I think the game checks the latest expansion Storm of Zehir's cd-key, so if you only owned vanilla Neverwinter Nights 2, there is really no point in changing it.