You would need to add a pump and hopefully find what is called an indirect water heater. This is a tank that has two internal parts: one for the water you use, and one for the heating circuit. You'd hook your tankless system to a pump that is controlled by an aquastat (a water sensing thermostat). When the tank's temperature was below where you want it, it would turn the pump on, this would trigger the tankless system to start heating the water. THis heated water would then go to the coil or jacket of the indirect, and transfer its heat to the tank. When the aquastat sensed the temp was warm enough, it would turn off the pump. How well it works would depend on how hot the tankless system can get and whether it is designed to run long enough to heat the tank. You'd size the indirect WH for your expected use. In theory, you might have all night for it to reheat the water. The indirect (if you get a good one) is very well insulated, and once hot can retain that heat for possibly several days depending on how cold it is around it. If you used up all of the hot water in the tank, it might take a long time to reheat it, especially if you can't set the tankless temperature very high. You'd need to size the pump so it allowed the tankless to reach it's maximum outlet. For safety, you'd also need some valves to drain the system, fill it, and an expansion tank, along with a safety relief valve if you want the whole thing to be safe. There are other ways you might accomplish what you wish and you may get some options if you stick around for awhile.