Original poster here.
My intention was to create a SchLib from the active schematic in order to copy and modify a particular schematic symbol. In Altium this is how I've always done it, unless creating a schematic symbol from scratch.
Digging deeper in the CS docs, it looks as though, in Circuit Studio, you can simply copy a schematic symbol from a SchDoc and paste it into a SchLib, which would be very convenient. Will try this when I get home this evening.
See CS reference below:
Notes
•The component that you paste into the active library document can originate from either a schematic design document or another schematic library document.
•If multiple components have been copied to the clipboard from the main design in the Schematic Editor, all components in the selection will be pasted into the library document.
•If the same component is pasted into the library more than once, or if more than one new component is added to the library without renaming, the copies are distinguished by the suffix _1, _2, _3, and so on.