True, a panel does not accept another panel - not normally, anyway. It is possible, however, to create two panels on the same form, one as a 'parent', the other as a 'child', then use formlib.ChangeParent to cause the child panel to become subordinate to the parent panel.
I commonly use this approach, and add a scrollbar to the parent panel which causes the child panel to move up and down. It allows me to put a great many more controls on a single form than would fit otherwise.
Gary