Please Note: Supplemental Feeds are only available in Google Merchant Center
Supplemental feeds provide additional, or supplemental, data that can be connected to existing product data in the primary feed. Supplemental feeds cannot add or remove products, or be used as a standalone feed; instead, they're used to update existing product data. Supplemental feeds can provide additional information to multiple primary feeds.
To use a supplemental feed, connect it to existing primary feed through the id attribute. Supplemental feeds will only update your product data when the supplemental feed contains IDs that already exist in a primary feed.
Recommended: Use Google Sheets
Google Sheets is the preferred format for creating and uploading Supplemental Feeds to Merchant Center simply due to convenience.
Start out by creating a new spreadsheet in Google Sheets
ALWAYS: The ID column must be used every time
Use all other columns for data to be modified or provide additional attributes
Below is an example using Custom Label 0 as the attribute to be "added". Once uploaded to Merchant Center this will supplement your primary feed with the data you have entered:
Uploading Your Supplemental Feed
Log in to your Merchant Center account:
Click Products then Feeds
Click on Add supplemental feed
In the next section name your feed, choose Google Sheets, then Continue
Click Select an existing Google spreadsheet in the next menu
Recommended: Create an upload schedule
Click Continue
Select the Feed and appropriate Country/Language
Click Create Feed
Last Steps!
To complete the setup of your Supplemental Feed, click on the feed name after the above step:
On the next page, click Fetch now:
Google will typically process your changes shortly after completing the fetch.