Product sorting in nopCommerce is not very dynamic and it is implemented using enum. You can find ProductSortingEnum under Nop.Core.Domain.Catalog namespace.
In any case if you want to add Best Sellers option to enum then you must customize code to implement it. In case if you want to do it from a plugin, you would have to make your own enum or DB driven table and override all methods where this sorting list and its logic is implemented in code.