The first course of action is to contact OneForAll at
www.oneforall.com (be sure to type in the www). On the FAQ page there is a link to contact them by email. They may be able to provide you with the correct 5 digit EFC which will allow you to program a button to perform the Line function.
If that doesn't help, an upgrade file at hifi-remote.com for the XV-DV313 may be similar, and it lists a function called "line 1 2". Using setup code 1384, the EFC is 05318 (or the equivalent 70854).
I don't know exactly how to program an EFC to a 7960, but you should be able to extrapolate from the 993 VPT command shown in the 7960 manual to use the 994 command for entering EFCs. Here are
general instructions for 9XX commands, but these are probably slightly different than the correct procedure for the 7960. You'll likely have to adapt a little bit. Of course, OneForAll may be able to tell you the exact procedure, so I'd start with them.