Look at the chemistry of the steel casting. Calculate the carbon equivalency and minimum preheat per the method listed in the Annex of D1.1.
As long as the chemistry is favorable and sufficient preheat is used, welding it to A36 should be fine. I would match the electrode to the lower strength base metal. Typically a low hydrogen electrode that has been properly stored should be used and should work just fine.
Without more explicit details, a more detailed recommendation is nothing more than a roll of the dice.
Al