You don't. The point is to charge based on risk. There is no sense in which you can transfer gains from one set of customers to somewhere else. The profitability of any group of customers depends only on the price you charge them.

And btw, lots of insurers specialise in pricing high-risk customers. If another insurer comes in and tries to subsidise low-risk customers using high-risk customers, then a specialist insurer just comes in and undercuts them profitably.

Even a low-risk customer becomes a bad risk at the wrong price. It is all about the price.