Is this possible and, if so, where would I find them?

The only system that comes to mind is the <span class="podkreslenie">Player Resource Consortium</span>. There is a rather interesting development, involving executable hooking as I understand it, in the form of <span class="podkreslenie">NWN Refreshed</span>, but that has not yet been released.

they are planning (somewhere in the future, possibly a year or so from now) to add new base classes to the server where I like to play. not sure how they do it, but the developers are masterminds when it comes to coding/hacking..

be warned that the PRC is a bit.. clunky. All the new systems they implemented work with dialogue based controls. For example, instead of a GUI to select and ready your manouvres, you need to work your way through a lot of text+choices. But the pack still undeniably adds a lot of value.

EDIT: though if NWN refreshed is released, it seems it will be truly, totally amazing.

I believe the means of doing so was planned to be through the same manner in which we do our engine hacks, by hooking the executable and redirecting to custom C++ functions outside of the game, similar to how NWN Refreshed appears to be handling it. Needless to say if NWN Refreshed is released in the near future it will save Acaos a good deal of development time.

While the PRC is likely the only source for new base classes at the moment, working around the engine in order to implement those classes does cause the result to be less streamlined than vanilla NWN...but if the added classes are all you're concerned about, the hoops you may have to jump through to use them could be acceptable.

From what I'm recalling now, Bioware made it "impossible" to add new base classes by default, right?

Base classes, as I understand it, are hardcoded into the game. This means either modifying the executable (illegal) or redirecting functions outside of the executable (legal).

The player resource consortium is a little clunky but it's waaaaay worth it just for the MASSIVE amount of content they've implemented.