Can I make crackling from cured ham skin?
The short answer is yes, you most definitely can but with a few caveats. First, if the skin has any significant amount of sugar on/in it from a glaze or cure it will burn fairly quickly and likely impart a bitter flavor before you are going to achieve the proper crisp a crackling is known for. Also, with hams, there is likely going to be a ton of salt in the skin to begin with and this is likely going to be intensified when you fry it up so be aware of that. 

I've never had good luck baking pork skin for crackling since the texture is never the same and it takes a lot longer. Deep frying is a much better/authentic way to go.