Add a custom Ribbon group to an existing custom Ribbon group in word 2007

Question

Hi

How can I add a new group to an existing (3rd party) custom ribbon tab add-in? I know that I can add to out of box ribbons by specifying the Tabs idMSo value but how do I do that for a custom ribbon tab. I have already tried the ID value of the custom ribbon, but it juts duplicates the ribbon? have also tried idMso and idQ attributes passing in the required custom tab Id but no success.

I can add it to out of box tab group by specifying the idMso value but not to custom tab

regards