Edit the Display Name for a contact in Contacts.app
Hope this may help you somehow. Download Bento for Mac from here. Unfortunately, it's not supported by FileMaker anymore, but anyway it works fine even on El Capitan (I use it here :)) Open it and it will ask permission to access your Contacts. Allow it. This it will open very convenient interface for mass editing your Contacts in list and/or individually with ability to add your own fields. Cheers.