Sure. In System Preferences > Users & Groups, control click or right click on the new account and choose "Advanced". From there, you can change the home directory.
Make sure to first copy the home directory to the external drive. I strongly suggest using the "ditto" command to make sure all of the ownership and permissions are correct. You also might want to do "Get Info" in the Finder on the external volume and make sure "Ignore Permissions on this Volume" is off.
This is the closest Apple Support document that describes the procedure: Change the name of your macOS user account and home folder - Apple Support. It isn't quite the same though.