Standard D&D Driders are former 6th level Drow that have been changed completely into a totally different 6HD creature via curse by Lolth as punishment for failing one of her "tests". They are no longer the same creature at all. They didn't adventure or otherwise gain any XP as a Drider to become a Drider anymore than a Human PC has to gain XP in order to become Human, level drained, diseased, poisoned, etc.
A Drider is a type of Aberation. Like a Drow is a type of Elf and a Dog is a type of Animal. A Dog can "take" levels in Animal but it can't take levels in Dog because Dog is not a class. So a Drider can take levels in Aberration or lots of other classes, but, not Drider because Drider is not a class.
Drider advancement is by class with a +4 ECL. A Drider PC begins it's PC life as a 6th level Aberration with +4 LA like a
Hill Giant PC begins it's PC life as a 12th level Giant with +4 LA.
If you want to use the class mechanism to compensate for the fact that NWN2 does not grant more than 1 HD at level 1 then you should make it a level 6 class with all the goodies granted at level 6 so PCs have no incentives to skip any levels.
Or a feat granted at level 1 that grants 6 levels worth of bonus hit points, saves, and all the rest. But, you can't grant BAB like this, and other problems.
Here is a Drider character:
http://www.wizards.com/default.asp?x=dnd/re/20030127a. Looking at the spell list you will see that she has 7th level cleric spells but only 7 levels of cleric. That's because Drider gives 6 levels of spellcasting.
You will see the same thing if you examine the Drider Sorceror in the
War Band Generator. It has 6th level sorceror spell casting but d8HD and 3/4 BAB.
The Drider Template does not help because it augments what is already there without adding levels unless the victim is less then a minimum HD. That's not a class.
The D&D Drider is a bad choice for a PC in NWN2 as NWN2 simply does not support the mechanism needed for one.