I'm not sure, if I understand everything correct, but you want to show a specific RibbonGroupBox when its module/assembly is enabled/active? So, you can define the RibbonGroupBoxes in the Ribbon control and just change their visibility (collapsed/visible)
according to your selected module. Or you can define the RibbonGroupBox in your assembly and add/remove it dynamicly to the RibbonTabItem at runtime (code behind/no xaml).
Hope it helps