Or you can copy and paste a drawing on a new frame by using the marquee / selection tool. And then moving the copied drawing while it is still selected, while using the onion skin.
Now if you want some kind of “tweening”, that is, automatic inbetweening from some drawing A to a drawing B, I’m afraid that’s not implemented in Pencil2D. For that kind of functionality I’d suggest trying out Synfig (http://www.synfig.org/) or Tupi (http://www.maefloresta.com/portal/)
Both software apps are free and open source like Pencil, but they are geared towards animation that use vector interpolation.
That doesn’t mean you can’t mix all the software. For example, you can keep hand drawing on Pencil2D your animation and when you need a complex motion, you can create that into the other apps and then composite it together (that is use a different video track to blend one layer on top of the other).
I’d also suggest you use the latest development update of Pencil2D to ensure that you can export your animation properly through image sequences or video.