It's even easier to model organic models in zbrush, than to do it in a 3d app.
just get a sphere and start sculpting
The problem is what you want to do with your model.
If it's for a game, you can make the model with zspheres in just a couple of minutes, then you export it to your 3d app.
In the 3d app, you add a good edge flow for animation and reduce or increase the poly amount to what you want your ingame model to have, then you export that back into zbrush, and start to add details for your normal map extraction.
unless you can model heads in it with lines similar to splines, but that i didn't see if any
Using splines to model a head, is nothing I would recommend, use subdivided polygons instead.
i'd recommend that you learn box modelling, go to kurvstudios.com, sign up for their free tutorials, and get their new box modelling a head video tutorials for Silo. Even if you don't use Silo, the concept transfer to any package.
If you want to try Silo, you can find at http://www.nevercenter.com/