3 ways to do this.

Adding the eyes as smart props to the interior of the existing head and creating master dials that dial in the eyes in various configurations with morphs that create the cavities to house the eyes, if necessary. The transforms for the eyes could be controlled by additional dials. Doing this as props might pose some challenges creating the crosstalks. Maybe not. I say might because typically you would...

Do this as conformers. Although parented props behave very much as conformers.

The conformer could be an entirely new head or just the eyes, as above. Crosstalk can be made to work in either direction. You can drive a dial in the host figure via a dial in the conformer, and vice versa as typical.

Or you could do this via adding geometry switching to the host figure. Very tedious bit of cr2 editing here. Ajax's old EasyPose Underground has features to aid in setting up geometry switching. I've only ever set up switching in one instance, in that case doing so via text editing for geometry switching fingertips in conforming hands. Not for the feint of heart type task. I didn't have EPU at the time. Unfortunately EPU is no longer available, it's a standalone app. Seems to work fine in win10. Back in the day it used to go for quite a bit of money (over 100 dollars IIRC) at RDNA, and does quite a bit more than just cascading ERC. Anyway, geometry switching creates a dial that calls a new obj to replace the existing actor. While the rigging remains the same for that actor, or at least the parent in the case of a conformer, it need not be the same vertice number and can call on it's own set of morphs. The new geometry must match along the actor weld seams however, if applicable, and you want bending.

None of these are what you would consider easy solutions, mostly depending on your customization comfort levels.

Geometry switching conforming hands... circa Poser 8