If you want to have a fitment table shown on your Storefront in a table and don't want to manually edit the Additional Content field on an individual basis, the following steps outline how you can use an eBay file only import to get that information easily and upload it into your account:
1. Go to Settings > Channel and navigate to Import section under the eBay Options
2. Make sure Import, Skip Other Channels, File Only and Exclude Descriptions & Data options are checked. The full selections should look as so:
3. Click Save Settings
4. Check back to see if file only import is done processing. When it is, it will not say Pending after Import and look like the screenshot above again
5. Click Download next to Last Import
6. Remove all columns except action, guid and content
7. Under content header, filter out anything that has Blank cells since those won't need to update
8. Copy and paste existing rows into a new CSV
9. Submit CSV skipping all channels
Please keep in mind that the compatibility needs to be selected and pushed to an active eBay listing in order to get the information in the content field. If there is an item you don't want on eBay for some reason but it's on the Storefront, you'll need to manually adjust the Additional Content field in the product editor or via bulk.