They can become natural, yes. But how long it takes depends on the player versatility. Just keep training and playing him in the position, eventually he will get there. From my experience, it may take 3-4 seasons in extreme cases.
no, he can definitely be a natural but within reasonable constraints. For example, you'd be wasting your time trying to train a centre back as a striker. But I'm going to assume you want to train a midfielder for different roles? or a striker as a winger or something? In which case, you can most definitely make them 'natural' in a particular position.