No, the Stories with Santa add-on tickets are only available as pre-book online, subject to availability on a first-come, first-served basis. You must have a castle entry ticket to access Stories with Santa.
Book Stories with Santa online through the Warwick Castle website here. You will need to select ‘Castle and Skate’ or ‘Castle Only’ tickets at the bottom of the Christmas page and you will be given the option to add on a Stories with Santa ticket for £4 per person. You will be asked to select a visit date and timeslot at the time of booking.