How do you skew a sprite?

I dont think so.

But you could use the drawing canvas and create the points with polygons then fill it in.

There is also the "funky quad" plugin. https://www.construct.net/en/forum/construct-3/plugin-sdk-10/funkyquad-released-plugin-expe-141190

If it needs to be in a sprite you could create it with drawing canvas, save it, then use the sprites "load from url" to load it into the sprites frame.