If you can do that, then you will be the only person on this planet.
A trick I read in Holland to get a custom WB is putting a white tissue before the lens, aim at the lightsource and use that as the WB. There're also expensive tools for sale, but they work the same. The basic idea is the neutral grey idea.
With taking more shots you will have allways the risk that 1) the exposure, 2) the focal distance and 3) the WB will change.
George