It's not clear if the OP is asking about revenue or award tickets. For revenue tickets, the ability to have a stopover is governed by the fare rules. From what I've seen, a general rule of thumb is that the cheapest fare classes have the most stringent routing rules and don't allow stopovers. As you move up, you might see a given number of stopovers allowed with a flat additional charge for each one, and then a given number at no charge. For instance, most of the UK-US T fares I've seen don't allow stopovers, but even on a U fare you can add one for £75. Sometimes the cheapest thing is just to price as multi-city with a fare break at your desired stopover, since the cost to get to the fare bucket that allows a stopover is more than breaking the fare and taking two cheaper fares. Easiest thing is to put it into multi-city on DL.dumb and see what comes up.