Hi, Knightwalkr,
The answer to your question really is no. There have been some DA revolvers over the years that resemble single actions, like the High Standards built on the Sentinel design during the first "cowboy" craze.
The granddaddy of them all is the Colt Model 1878, sometimes called the Double Action Frontier. Except for the grip and trigger guard, it looks a lot like the SAA, and the barrel, ejector rod and ejector rod housing are interchangeable with those of the SAA. One in decent shooting shape will cost around $2000-2200, though, and they tend to be fragile.
But to convert a SA revolver, like a Ruger Vaquero or a SA Colt to DA, nope.
Jim