The structural cabin design wouldn't accommodate a full glass roof (hey if you throw enough money at something or skills anything is possible). The cabs on these trucks are like cardboard boxes and flex around and would be worse if it wasn't for the chassis frame. You'd need to cut the entire roof off, create recessed channel frames over the tops of the doors and across the front and rear glass. It will need a custom made safety glass turret that would then have to be glued in with a urethane sealant and it would also probably need provisions for drainage channels. Theoretically the added framework would be enough to reinforce the cab from flexing and the glass will then become a structural part of the cab construction once the urethane has a hold on it. But if it isn't enough and the cab wobbles around, that glass could pop - kinda scary. Gluing in windscreens is how manufacturers got away with making the bodies on cars less rigid in the late 80's - the glass became a structural feature of the car.

*Fun fact - Peterbilt truck cabs are completely glued together