As long as you only do classic weighting, rigging the eyes is easy:
- Take care that the eyes are separate mesh parts. You can either separate them as submeshes in your head object, or just make separate objects for each eye, it does not matter
- Take care that the eyes are rigged to your Avastar. If the eye meshes are submeshes of the head, then they are already rigged together with the head. if they are separate objects, then take care to bind them to your mesh as usual
- Take care that the entire eye mesh of the left eye is only weighted to the mEyeLeft bone
- Take care that the right eye is only weighted to the mEyeRight Bone
Now export as usual. Things should work fine for you.