Sounds like you'll have to go in the backdoor if the Edit function won't work.
Go to the phone keyboard screen and punch in the number you want to add. Right underneath the number you typed will be an option to Add to Contacts. Tap that area and there will be another screen pop up that gives you the option to "Update existing contact" or "create contact". Choose the Add to Existing Contact and scroll to whom you want it added to. Done!