Yes, For NPCs, Level and Hit Dice should be synonymous.
When coding for NWN, if you need to know the Level of a monster, you check their Hit Dice.
When looking at how challenging a monster is, then we look at its CR (Challenge Rating) instead. In table top when you "advance" a monster, typically it gets 1 or more hit dice, possibly a size increase, and its CR advances. There's typically a ratio so that the CR doesn't advance as fast as the monsters actual Hit Dice, which is why you can see, say, CR 15 Giants with 20+ Hit Dice.
In table top, adding a PC class to a monster *typically* does increase its CR and Hit Dice 1 for 1, which is a way of saying that PC classes, picked correctly and played to maximum effect can be pretty powerful.
But again CR is only a calculation to help pit appropriately powerful monsters against the correct level of PCs. A CR 15 creature in table top is 'a challenge for 4 PCs of 15th level', using 1/4 of the total of all those PCs resources to fight. (Probably not entirely true in NWN, but that's the ratio in table top). From that expectation, we could expect a single CR 15 monster to fight a single 15th level PC, and use 100% of that PC’s resources (which means maybe the monster wins, or maybe the PC - they should be approximately equal).
So, for monsters, their level is always their hit dice; advancing them or adding class levels gets them more it dice and thus a higher level, but its CR you generally watch to determine if the NPC is the right challenge for what you are looking for.