You can add SSS to eyewhites by choosing the eyewhite material (sometimes called eyeball or "sclera", but depends on the model) and select the "Create SSS" macro (I'm using P6, so I don't know how much it's changed).

You'll probably want to experiment but I change the FastScatter color to 50% grey but leave the other colors at their defaults, since a real eye has blood and stuff underneath the surface. Perhaps someone with more experience can offer their suggestions.....