Can a person have an external av fistula?

Yes- not practical: It turns out that the first "fistulas" made years ago when dialysis first started were "external" fistulas. They were actually synthetic/plastic tubes connecting an artery to a vein. There were significant problems with bleeding and infections. These "external" fist las are not used today. If someone needs urgent dialysis they get a catheter. For long term dialysis either a fistula or a graft.

Not really: A catheter is external. There was an actual external fistula used more than 30 years ago, but not used now. It would be nice not to be stuck all the time.