adding a subgroup to a group with CCK?

Hi,

I'm creating several CCK content types. Some of them will have fields grouped within collapsible blocks. No problem with this (I do this from the 'add group' tab).

But I need to create some collapsible subgroups within a collapsible group. CCK only allows me to create groups at one level, but I cannot find a way to create nested groups within the same content type. How can I achieve this? do I need an extra cck module? which one in this case?

Thanks everyone for the tips