That's actually a very good question. I have the same ttable myself, but I've never thought about that. I don't have any problems with the grounding though. If it's not a problem don't fix it, cause there's nothing to fix in the end...
correct me if i'm wrong but you're probably trying to connect your numark pt to a mixer if the numark is anything like my vestax portable (and i'd bet dollars to donuts it is) there isn't a grounding wire on it as it was designed to be a stand alone device. instead of connecting it as a phono input try using it as a line input.