Unfortunately, that doesn't seem to work always. Some of my recent experiments with heads have shown that if one tries to override base Bioware heads with new ones then the models do override but the textures do not. Naturally, because the new head model is using the wrong texture, this almost always looks awful. See, for instance, this example of my attempt to modify female human head 004. The image on the left shows the head in a non-Bioware slot (hakpak), whereas on the right it uses head slot 004 (override). Even though a new texture for that head was placed in override, NWN still used the core Bioware texture.
Ultimately, if you want to replace existing Bioware heads, it looks like you have to take the hak route. Override just won't cut it.
As I put that KotOR Heads hakpak together, by the way, please feel free to PM me any questions you have about it. There are some quirks to the way the game treats heads that I've discovered during my work; they might help you.
