Select all verticles, duplicate them, symetrize by "Global X", join the two halves (you can also use Mirror Modifier in modifiers stack).
When the main model is ready, you should do UV Mapping. In Blender it can be done verysimply and fast. Just mark thee seams like shown below, go to UV Face Select mode, press [U] in 3D View and choose "Unwrap".
Export the model to (Wavefront-) OBJ file (if you have the Linux version, download the script from Blender.org -> Python
Scripts). Import it to ZBrush, check the mapping ("Uv Check", then "Fix Seams").
Now it's time to give the head its basic shape. Multiply the polygon amount by pressing "Tool -> Geometry -> Divide" and use the "Inflat" tool ("Transform -> Inflate"; in "Edit" mode).