Customizing Punctuation Groups

I'm trying to add a custom punctuation group. I'd prefer it to be modeled after the default "Some" group, with a few additions, like speaking dashes, braces, and colons. The problem is, those entries are already present and I can't seem to modify any of them (Voice Over insists the popup buttons are dimmed). And I really don't want to have to totally reinvent the wheel and create a group from scratch. Is there some way to create a group based off of the built-in groups and then edit the entries inherited from that group?
Thanks in advance.