During the setup, you have the option to skip any migration. I also think the installer will see you have a user setup already and allow you to skip over user creation. Worse case (as I don't remember the installer 100%), you setup a "temp" user. You can delete it later. It won't erase the user that's already there - just don't name the temporary user the same as your existing one.
Another option would be if you have an image or Time Machine backup, you could try and manually recover the .AppleSetupDone file. But, the above is probably the easiest way to get going again.