If you want to import a supplier to your products you need to map at least one supplier as your default supplier. To add more than one supplier you can use the "More" arrow beside Supplier to add as many extras as you would like.
Each supplier you wish to import must have it's own field in your CSV file, or it's own tag in your XML file. You need to provide the supplier name and this will be either mapped to an existing supplier, or if the supplier doesn't exist then it will be created, associated with your store, or multi-stores, and linked to your product.
Basic format for importing a supplier:
Import Supplier Reference
If you wish to import a supplier reference for your product, you must add the reference to your supplier field separated by a : (colon)
If you have your supplier reference in a separate column in your product CSV or XML, you can use the Step 3 operation "Append Field to Field" separated by ':' in order to create the required format.