You can add tags to your Products in bulk via CSV. From your Inventory page, click on the Bulk Manage drop-down menu > select Update Existing Products > click on No, I need to download a list of my products > select Product Tags under Product Attributes > Click on Build Product Sheet and Download. Add product tags for each product under the Product Tags column. If you need to add multiple tags for a product, separate each with a comma but no spaces. Save and upload the filled CSV.