Could you use a 9kg gas bottle as a boiler for a steam engine?

It all depends what steel the gas bottle has been fabricated from. It may not have been made with a steel that can take high temperatures. If you are designing a boiler then it should be treated as a pressure vessel and you should follow a code such as BS5500 or ASME VIII. If you can find out the steel used for your gas canister and check the dimensions and thicknesses are ok to one of the specs above then it would be safe.