Is it possible to add new fields?

I’ve found that I can add a new fields for a particular entry type by typing a new field name into the dropdown box in Customize Entry Types Optional Fields pane and clicking Add. For example, I added Printing, NumberOfVolumes and LCCN for the Book entry type. JabRef remembers those fields for the Book entry type but does NOT remember those fields in the dropdown lists. Is there any way to add my own custom fields and have them available for selection in the dropdown lists across ALL entry types? (It would be nice if I could just directly edit the list of fields without the context of the Customize Entry Type.)