Conversion of Instant tank-less gas water boiler 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