Can I convert a tank-less water heater to a tanked one ?

I have an instant gas water boiler that is difficult to mix hot water with cold water during the shower , plus in the cold winter I waste so many gallons of cold water down the drain until the hot water arrives to the faucet so it is not practical at all .

I want to connect an isolated water storage tank to the boiler and convert it to heat that water in that tank to a good high temperature , but the boiler is not designed to circulate hot water through its rubber diaphragm valve that opens the gas when you open any faucet.

Traditional boilers that work with storage tanks are NOT available in my area but only tank-less instant ones are available .

Please help and thank you very much in advance

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.

Boiler Diaphragm !

Thanks for your reply
I totally understood the heat exchange solution you suggest but you missed a very important point that I mentioned which is the rubber diaphragm gas valve that the cold water pressure coming from the intake cold pipe PUSH it to let it open the gas valve for the burners to fire , this rubber diaphragm is NOT designed to handle hot water but only cold water at the intake side .
But your suggestion depends on circulating the same water using an electrical pump operated by a thermostat which is fine but the boiler is not designed to circulate hot water but only take cold water from one side and deliver it hot from the other side ...

I hope I could get my point through to you and looking forward to hear from you

Yes, I understand that the unit senses flow to turn the burner on (this is true of all tankless systems). That water would run in a loop through the new indirect water heater, powered by the pump. By pumping it, that flow should turn the tankless on. It would return after going through the indirect WH cooler than it came out, and then be reheated in the tankless. This supply of hot water should transfer its heat to the new tank. You'd run your water to the indirect, and the hot would come out of this new tank. Now, whether it will work if the inlet is warm or not, that I'm not certain, but should. In some places, the inlet water can be quite warm. WHen I lived in Kuwait, the water tank on the roof in the middle of summer was too hot to touch all by itself. We had a tank water heater, and turned it off. We used that water that had eventually cooled as our cold water supply and the cold heated in the storage tank on the roof as our hot supply. In the winter, we turned the water heater back on.