I've just noticed that the series and number fields now appear for preprint manuscripts. Thanks very much for adding that.
So yes, the next question is how to make that information appear in bibliographies. I have defined my own custom style (to match my university's requirements), so I should in principle be able to edit my .csl file to add the series and number details when appropriate. However, looking at the list of variables in the citationstyles.org editor, it's not obvious to me what the variable names are that correspond to the series and number fields in Paperpile. Please could you clarify?