Here's a how-to article if you have already developed a B2B relationship with a supplier, but want to add some additional products.
Go to your B2B catalog (Settings > ECommerce > B2B Catalog)
Select Add Items
Click on the Supplier whose products you're wanting to add to your catalog.
Their list of available products will appear. You can check the box to add it to your catalog and select Edit to make changes to the options.
Save your changes!