You are on the right track.
If you specify on the category field the email needs to go to, then you could copy that value onto the Reminder to send it to a specific supplier for that category.
Someone requested something similar in our environment but then decided against it.
In our environment we don't have the facility to e-mail directly to an e-mail address therefore I was going to set the supplier up as a user with their e-mail address on the user record.
I was then going to Add a decision and a reminder box, we have a two stage assignment so my decision would have gone in after the 'with 2nd line' action.
The decision would check if the category matched the one required, if so, send the reminder to the user 'supplier' with whatever information/placeholders required, if not continue...
There are probably loads of better methods of achieving this but this was my simplistic idea for a quick implementation just for one supplier!