If you are managing vendors by category and looking for a way to handle vendors requesting to be suppliers for different categories then you can use the vendor category request process. This allows you to have a process to handle the approval optionally with a workflow process. Here is a quick walk through what the process looks like.

If you are wanting to control vendors by category based on approved or preferred this previous post might be helpful.

https://organicax.com/2015/05/08/purchasing-policy-category-access-policy/

Cheers

Lachlan