The first step is to create a parametric sphere in the document. Here I have also turned on the wireframe view so that we can orient the pole of the sphere forward. 
CREATING A PROCEDURALLY TEXTURED EYEBALL IN ZBRUSH 3.1
For this demo we will create an eyeball from the Parametric Sphere and then append it as a subtool into an already textured head.
STEP 1:
The first step is to create a parametric sphere in the document. Here I have also turned on the wireframe view so that we can orient the pole of the sphere forward.
STEP 2:
Next I will switch the material to the Toy Plastic MAT which has much more of a glossy eyball feel to it (alternately, you can make your own matcap material here). Then I will subdivide the mesh until it has a suitable resolution for polypainting, in this case, over 500,000 faces.
STEP 3:
The next step is to conver the mesh into a polymesh so that I can begin the polypainting process (parametric objects cannot be polypainted).
I will then Turn off Zadd, Set the RGB intensity to about 10
and under Tool-Texture I will hit the colorize button.
Finally, I will set the Draw size to a smaller brush (About a value of 7 in this case).
STEP 4:
Next I will enable Radial-Z Symmetry under the Transform palette and set the radial count to around 40 or 50 points.
STEP 5:
First, Fill the Object with White color
then start drawing with Black from the center pupil, outwards, creating a radial pattern.
With a bit more effort, and several brush strokes later, a pupil and iris starts to evolve. Try rotating your pen drawing outwards in short strokes to produce wispy patterns.
STEP 6:
Add in some color, in this case I used a blue tone primarily, but use compliments and analgous colors to bring more life into the image than one flat tone.
then start adding in white in some areas too to lighten up the pupil.
STEP 7:
One problem is that our material is not yet baked into the mesh. When you append this later to the head, it will take on that mesh's material and not retain the toy plastic. As we see here, switching to the Red Wax material no longer retains the eyeball look.
So, to get the material to stick, switch back to toy plastic, then change your draw mode from RGB to M (material painting only)
Under the Color Palette, Choose Fill Object with the color set to white, and the eyeball should take on the look of the desired material. Note here how after the fact, I have switched back to the red wax material and the eyeball still looks like the toy plastic is on it.
STEP 8:
The Polypaint can now be converted to a texture file by going to Tool-Texture- Txr>Col. This texture can be exported, along with the base resolution eyeball for use back in Maya.
STEP 9:
However, if you want to see your head with the eyeball in Zbrush, thats simple too. First Import or Load your head.
STEP 10:
Next, Append the Eyeball as a subtool
STEP 11:
In this case, my eyeball was WAAAAY to large, so I will resize it using the controls under Tool-Deformation, specifically the Size slider in XY and Z and the offset slider (in x,y, or z) to repostion it into place.
STEP 12:
With a little bit more tweaking, you should have it set!
