Can you fit TruDesign Ball Valves to bronze or Stainless skin fittings ?

Yes
TruDesign Composite ball valves can be screwed onto an approved bronze skin fitting.
First the thread on the bronze skin fitting must be identified to match the TruDesign Ball Valve (either BSP or NPS).
In the case of an old existing bronze skin fitting you must check that the bronze is not pitted or corroded from electrolysis and is in good mechanical condition.