If you had a printed artwork that used only Pantone Reflex Blue U, Rubine Red U, and 802 U (Neon Green) — some colours are pure, some are blends, some are at 100% density, others are less — and you were then provided with an RGB scan of this print (saying the unprinted paper happened to have an Lab value of 100 0 0), could you separate the RGB file into the corresponding spot channels (Reflex Blue U, Rubine Red U, and 802 U)?
Would you use the same method if there were 5 spot colours, instead of 3?
From what I can tell, converting to Duotone/Tritone/Quadtone would be unsuitable, because it requires converting to Greyscale first.
YES it can be done but it’s no easy task.
How difficult it would be depends entirely upon the artwork.
One would use a multi-channel Photoshop document and create as many spot channels as are necessary. Then, on the channel, you simply paint or create black/grey areas for the color. Or, recreate things in Illustrator assigning spot colors.
There’s no “automatic” method via a few menu commands to complete such a task. It would involve selections, and painting on channels, or creating objects in Illustrator. You couldn’t merely scan, run a few command, and end up with proper separations.