Currently you can only drop a plasmoid from Widget Explorer onto the Grouping Plasmoid. It should offer the same functionality as ContainmentInterface offering to create an applet based on the mime data dropped, e.g. dropping a folder should offer to create Folder View. Might need some more generic library function to avoid duplicating all of its logic in ContainmentInterface and the grouping plasmoid.
Hmm, it's an interesting idea, but I think that might be pretty unexpected, and also what other mimetypes would it makes sense to support? Probably simplest to only allow adding actual widgets here, I think.