Possibly related to Unreal Engine Issues and Bug Tracker (UE-48454). You can’t do (right click) Wrap With… on single widgets in the expandable area widget, so you’ll have to drag the widget out, drag the container widget you want into the expandable area widget, then drag the original widget back into the container widget.
Edit: Replace with… doesn’t work either. If you do so by accident, then move the container out and try to move out the children out of the container box you can end up with shadow widgets (like in https://issues.unrealengine.com/issue/UE-29987). If you end up with a widget you can’t delete (eg. TextBlock changes name to TextBlock_1 on deletion, isn’t deleted) and try to delete the container with the shadow widget still inside you can freeze the editor.