Can a Wagner Opti-Stain Plus High-Volume Low Pressure (HVLP) Handheld Paint Sprayer be used to paint a car with a single stage paint product?

I wouldn't even try. Your talking spray can results.

The Opti-Stain sprayer will produce a fine finish, but generally for automotive applications a very fine finish is what you would seek, so it may not produce the finish you are looking for. For materials, the Opti-Stain Plus can spray water and oil-based clear stains and sealers, some semi-transparent stains and sealers, enamels, and polyurethanes. Most materials should be able to be thinned and all should clean up with soap and water or mineral spirits.