Import new products as drafts
Set newly imported products' post_status field to 'draft' instead of to 'publish'. Only applies to products being imported for the first time, not to already imported products.
Create a custom plugin
If you haven't done so already, create a custom plugin.
Add custom code
Add the following custom code to your custom plugin:
Save your custom plugin file.
Now the next time your Product Set updates, any new products added to your site will be imported as drafts instead of being automatically published.