Yes, you can. In fact, there are prescritpion sunglasses that provide vision aids and eye protections for people who with poor vision. you can order it with your eye prescritpion. Aside from prescritpion sunglasses, There are also clip on sunglasses, transition sunglasses that can be made with prescritpion lenses. As long as you have current eye prescritpion, you can expected to get sunglasses with prescription lenses.
Of course, you can change your plano sunglasses to prescription sunglasses. plano sunglasses are also called non prescription sunglasses. You can take the non prescription sunglasses back to the place where you bought them and ask your optician to replace the clear lenses with prescription ones. Generally speaking, the lab technician will do it for you. But some lab technicians are unwilling to replace the lenses with prescription ones. Because you may be not pleased with the results. I would suggest you get some new prescription sunglasses instead.