Technically, the original storyline of Gothic 2 and the storyline of the Night of the Raven addon can be played in (almost) any order and may be even arbitrarily interleaved. The main part of Night of the Raven takes place in a new part of the island that is only accessible through a certain addon quest. Once you reach this part of the island, you may portal back and forth to the original part and progress as much of either storyline as you desire. The only point of no return is close to the end in the original storyline, namely when:
you are supposed to leave the island by ship.
After this point, you won't be able to access the Night of the Raven part of the island anymore.
As for right time to deal with the addon, unfortunately, I don't remember which level I have been. If I recall correctly, I started (and concluded) the addon storyline somewhere between chapter 3 and 4 of the game. However, when and how you tackle this quest is completely up to your judgment. Gothic 2 "tells" you that you are not strong enough to master your current quest by placing monsters in your way that are too strong for you in your current shape. So if you fail some quest repeatedly, it is a good idea to go and do something else until you have become strong enough.
Having said that, you should be aware that with the installation of the addon comes a significant rise in the difficulty of the original storyline (as compared to playing Gothic 2 without the addon). However, once you beat the Night of the Raven main quest, you will obtain a weapon that will make the rest of the original storyline almost too easy. But beside these balancing issues, Gothis 2 (plus addon) is really one of the best single player RPGs to play.