is it permissible for one to clip dark lenses onto regular glasses if the person would otherwise not be able to go outside due to extreme eye sensitivities toward the sun?
During the week there surely isn’t any issue with it. Regarding shabbos if there is absolutely no fear that he might take them off when he is outside, i.e. he will not remove them even if he is in the shade, or if it will suddenly get cloudy, then it is permitted. But if he might take them off when he is in the street then he is not allowed to wear them, unless he gets sunglasses that are actually attached to his regular glasses or prescription sunglasses, that can be worn as regular glasses.
Shmiras Shabbos Khilchoso 18-18, ftnt. 71, 73, Poskim.