[Koha] how to add a new copy of a field when creating a biblio

Hi, Can anyone tell me how to add a second copy of a field when creating a new biblio? For example, when you want to enter both the 10 and 13 digit versions of the ISBN, you need to duplicate the 020 field. Or when you want to add additional subject headings, you need to duplicate the 650 field with all its subfields. How can you do that in koha? Also, I don't use most of the item fields on a regular basis, and would like to hide them to make creating items easier. I tried setting the 'hidden' attribute on those fields to 7 and 8, but that didn't work. Any ideas? Thanks, Kim Auburn Public Library auburnpl@iowatelecom.net