You should be able to let players use virtually any race, and gain experience and input attributes and abilities and that just like regular player characters. The main problem is a lack of animations and armor/weapon models that work with other races. Wearing gear with an unsupported race may create visual anomalies.
Do you know if it's possible to add races to the character creation window? For example, in the case mentioned by Vith, could we add an orc race? Obviously customization would be non-existant and armor / certain weapon models would be problematic, but is it possible and, if so, is it easy to do?