The .123 file that is created by our PCB123 software is our own format and cannot be used by other manufacturers. You do have the option to purchase the gerber data from your PCB123 design. The following post explains how to do so.
PCB123 Design
If you are already using KiCad you can generate gerbers from your designs and submit an order for boards through one of our services at the . Please contact customer support if you have questions.