Format Strings

I have a newbie question which I can't seem to find in any of the documentation. How do I add format strings to my format string lists? For example I want to have a list of differing format strings in my convert filename from tag list. When I try and add a new one it replaces the existing one instead of adding a new one. So how do I add new ones?

Also, is there a single format string list maintained somewhere that I can put new format strings into and select from multiple actions?