Add group as sub-menu into GenerateGroup

I am trying to add a group of actions as items in a new sub-menu into the generate popup menu.
The actions are listed in the generate menu, but directly, that is without a sub-menu containing them. Adding that same group to the tools menu works as expected (with a sub-menu containing the actions).
Is there an additional attribute (besides popup="true") required for adding a group as a sub-menu into the generate menu?

thanks in advance for any advice on this.