It turns out not to be that hard, but the relationship among the game components and the various storage sites for Sims is hard to understand at first. To clarify, I made this graph to show how Sims can be moved around the various parts of The Sims 3 (click for full-size image):
To get a Sim from the Library into Create a Sim without using a mod:
- Put a copy of the Sim into a house, play the game as that Sim.
- Click on a dresser and select Plan Outfit. This enters Create a Sim in single Sim mode.
- In Create a Sim, click Save Sim to Bin.
The Bin stores single Pre-made Sims. Towns, the Library, and the Clipboard in Edit Town mode all store households of Sims. The Clipboard is temporary storage; apparently Sims stored only there can be lost if not saved elsewhere before quitting Edit Town mode.