Why would you want to go from the newer app back to an app which is no longer supported?
In Fresco you can import images created in Sketch, but you can't import from Fresco into Sketch.
On the Import and open screen in Fresco, select Sketch or Draw project, on the next screen select Draw or Sketch and the images created in that app will be available to open in Fresco.
Exactly. Sketch is pretty much an obsolete app. And Fresco is a powerful app and only getting better