Why do you need a built-in property for that?

We regularly creates our own BrandContent, by inheriting from NodeContent, and use them as categories.

Optionally: If you want to you can add your own ContentReference property to your product content type with BrandContent as AllowedTypes, that way you can make sure there is only one brand and doesn't have to go through its parents.

You can read more on creating your own content types and see samples here:
https://world.episerver.com/documentation/developer-guides/commerce/catalogs/catalog-content/