You can but only if you are in the view and give the frame and they make the measurement of the lens for it.?Then they are measured and the lens in accordance with the size of the frame.?But do not buy cheap plastic frame just a hint.
Maybe you can buy the frame and then ask an optician to cut a pair of lenses for you. But not all the opticians will do that for you. So you'd better bring your frame with you and then ask the opticians store by store. Maybe some guys will do that for you. Good luck!
I have heard that some opticians will cut lenses for the frame. They are glass lenses. But I have never heard about that they will cut sunglass lenses for a person. maybe you should ask them fist. We often buy sunglasses in the eyeglasses stores. If we don't have any vision problems, we can choose a pair of plano sunglasses in their stores. But if we want to buy a pair of prescription sunglasses, they will do that for us. Maybe it will take a few days to get that if they are very busy.
Though some opticians will do that for you, I still think you'd better buy a pair of sunglasses other than buy frame and lenses separately, because this is very inconvenient and this will not save your money. So why not just ask the optician to make a pair of complete sunglasses for you. Fake frames often have very poor quality. Hope you can take this into consideration.
If you want to save your money in this way, I think it is not a very wise way. But if you just love the frame of that pair of fake sunglasses, you can do that. However, the quality of the fake sunglasses can't be guaranteed, so you may waste a pair of good lenses if you broken your frame in future. It will be difficult to find a pair of frame to store your lenses at that time.