Unfortunately there is no simple way of doing this.
You could export the sketch as SVGs and then open the copper layer/s in an SVG editor (Inkscape etc).
Then you would merge all the little pieces into a solid.
After that you would have to clean up the outline using something like Inkscapes Path/Simplify.
Once you had a nice clean outline of your traces you would crop that image (using an intersection with a square or whatever shape you wanted).
Once you had it exactly as you wanted it you resize your page to match the image.
Now ungroup until you know the image is no longer in a group.
Now put the image in a group and change the group id to copper1.
Then you would turn off all fill and stroke from the image leaving you with an invisible image.
Save that image as a plain SVG.
Open the example from the other thread with the Fritzing parts editor.
Save as new part.
Go to pcb view and change the image with the one you just created.
Change the meta data so you know what the part is since it is invisible.
Save the part.
Use the part to expose the copper.
You could also add a copy of the SVG with white fill to the silkscreen layer by putting it in a group in the SVG with the id silkscreen. This would let you see where you are placing it but would be cropped on export.
There are a few videos around on making parts as well as lots of tutorials on SVG editing.
If you are using Inkscape I have a list of tips and tricks Inkscape Tips and Tricks