While there are certainly lens magnifiers available, I don't know if any will offer the level of magnification or detail you'd need to view the retina. I've seen some that go as high as 20x but having never tried them, I can't say if it'll work.
Doesn't seem like a phone camera would be ideal for that kind of thing though, especially considering what a niche audience it would target and the fact that it would have to be redesigned every time the phone went through a redesign.