You can do this since v8.3.0 by defining a property in the widget xml of type=”widgets”.
See the docs section 3.3. Widgets
At the moment, you cannot nest pluggable widgets (or the older custom widgets). I expect that we’ll see this capability at some point in the not-so-distant future, as it would open up a ton of possibilities for widgets.
It is talked about in the presentation at Mendix world
Maximizing Next Generation Experiences with Mendix Pluggable Components
It not there yet.. would be love to see it
thanks for the answers!
It will be really cool if we get this feature at some point. Hopefully soon!