[Koha] Koha to MARC mapping for field not in database

Hello, What do I do when I am trying to map a MARC field to a field that is not in the koha tables? For example, I want to map 306a Playing Time to something but there is no suitable match (such as biblio.playingtime) in the mapping fields list. I am also curious about 650a Subject/Topic. Am I just missing this when I look for it in the database? In the end what I'm trying to do is show the Playing Time on the OPAC. I understand this may mean a change to the template, but I've added other fields and now I want to add this one too. We are using Koha 3.00.02.012 with MARC21. Thanks for any help, Melisa Canales