Some frames yes, some frames no. Really depends on the make and model, and even current condition of the frame.
What it boils down to ultimately is if you buy it, you can do whatever you want to it.
But there are those here, if you are looking for approval which you shouldn't, will jump your $H!T if you do convert something that they deem you should not; its your asset do what you want to it.
Nobody is going to cry foul if you convert a 1977 Schwinn Super Le tour in crappy condition that is just a frame and fork.
But if you happened by luck to get your hands on a MASI torched by Mario Confente and converted that. Well some folks would want to shoot you in the head.
Just talking about bare frames up there
Down here on the other hand, if you have a fully stock bike in decent condition that has original parts from way back when and you want to convert it, some may say you're looney.