No, there is no right-click indication for adding as a widget, but it still works when dragging an app somewhere outside of whichever task manager is being used. Drag one to the left of it, say just to the right of the Launcher icon, and hold briefly for any widget (like the task manager or the Pager) to slide over a bit. if you don’t see it slide over, the dropped icon pins to the task manager if it is located you are dropping the icon.
This iirc is similar to how drag-and-drop worked on Plasma 5. I think.