There are a couple of ways to include a supplier into an existing supplier panel. These are detailed below.
When viewing the details for the Supplier Panel there is a link next to Panel Members to Add Supplier that will allow you to search for a supplier to include in the panel. There is also a Quick Link button to Include Supplier that does the same thing.
When viewing the details of the supplier there is a link next to Supplier Panel Members to Add to Panel that will allow you to search for a Supplier Panel to include the supplier in. There is also a Quick Link button to Add To Panel that does the same thing.