When you encrypt your startup disk with FileVault, macOS gives you a recovery key. Does macOS also create one for external drives? If not, would it be possible to make it create one?
The short answer is no.
When you use FileVault, you are encrypting the entire boot volume, not just a single volume or folder.
If you're looking for something where you can encrypt a removable volume (i.e. USB drive), take a look at VeraCrypt where it allows you to have both password and keyfile authentication. It's also cross platform which is definitely a plus.