Most analog scopes can do one-shot, but you would use a scope camera with Polaroid film (or digital camera) to record it. There were also analog storage CRTs that could store the signal for a while. If you're interested in those, I'm sure you can find some Tektronix documentation. You're not going to convert a normal CRT into a storage CRT, however. You can find analog storage scopes used, but they're obsolete technology and pretty much inferior to digital storage oscilloscopes.