This is gonna sound so dumb, but...
Make a photoshop brush of them. Draw your glasses from different angles and make a brush set out of them at a HIGH resolution size. Then BOOP! Glasses are on! If they still look wonky use the transform, perspective or warp tool to get them in the right place (talking with photoshop experience here, so your mileage may vary).
Of course that depends on how simple the design of the glasses are. I'd suggest keeping them one color and adding highlights to the lenses AFTER you put your brush down. I wish I had something like that for the dude in my comic who wears glasses. His are a pain in the butt to draw from so many angles. :/