Click the "load saved preset" button just to the left of the "Add Critter" button and you'll be able to select any .critter file you want as the critter to be used by the Add tool.
There are a lot of other ways to add existing critters to your sim. In addition to adding one at a time, you can:
- Use the "load preset" button in the simulation settings panel to change the default spawn to a saved critter
- Use the "merge pops" button in the main menu to bring the critters in another save into your current sim
- Use the "spawn source" option in sim settings to select a save, or save directory, to use as the source for critter spawns instead of a random or specific critter
- Use "micro-merge" to periodically merge a small number of critters from a directory of Autosaves