Hi,
you can assign custom table to any site. In custom table settings on a side navigation there is options Sites, in here add sites that are needed.
We have used both options for global data like social sharing, default URLs... etc. We like more Settings because you can add categories and it can have some structure. Both ways are ok. I don't think you need custom module for this.
Here is how to add custom settings:
https://docs.kentico.com/display/K8/Adding+custom+website+settings