Yes you can, but bacteria colonize better on surface area. It may be a better idea to grab something solid from that pond. Also, it depends on how clean it is...if you add pond water, you risk introducing contaminants into your system.
I added some scrapings from my filter media from mine and it seemed to help some as I was fully cycled in about three weeks.
I think it should work. I would worry about controlling algae as well as surface water and such getting into the system. I'm sure there are others who are using a pond, you may want to try another forum.
Don't float rafts on the fish pond....The fish like to nibble the roots.