Yes, there is a battery clip adapter. All of the major manufacturers of "universal" power supplies offer these adapters - Visual Sound (1 Spot), Godlyke, Voodoo Lab, MXR, and so on. Some supplies come with them, but even if it doesn't, they're really cheap by themselves.
It's not difficult to add on a real DC jack, but most people prefer not to modify their pedals, especially if they plan to sell them later. Some pedals which lack DC jacks can be quite valuable, as it's something you often see in old rare pedals. The black Russian Big Muff is an exception because it's really cheap.
Not to mention, you'd need to find a good place to add a jack, drill a hole in the chassis, solder in the necessary wiring, etc...