Is it possible to "dope" glass to change it's refractive index?
Glass doesn't crystallize at all, that's the point. Consequently, unlike crystalline materials, it doesn't have fixed composition. In a way, it can be said that glass consists of impurities. And yes, of course, you may change its composition this way or that, so as to tune the refractive index to your purposes.

Further reading: crown glass, flint glass.