Add Google promotion_id to woocommerce products (admin only)

I’m trying to figure out the best method to add a Google promotion_id field (admin only – not visible on site) to each of my WooCommerce products so that I can include relevant Google promotion id’s for eligible products in my Google Merchant Centre product feed. I’m using Export Feed to manage the merchant centre product feeds and they have told me that I can add any custom field or attribute to the feed. One additional challenge is that I ideally want to be able to upload the unique promotion id’s from a csv file to save me from editing each individual product (I currently use wp all import as my product upload tool). I initially tried using a plugin called WC fields factory. I was able to create a custom admin field for the promo id but ran into two problems – a. Didn’t seem to work for product variations; b. WP All Import could not see the new custom field to import to it. I’m looking for some guidance to help me do either of the following:

Option 1. a. Create a custom admin field for all products (simple and variable) that I can populate with the unique google promotion_id’s I generate; b. Figure out a way to import values to the new custom field using WP All Import

or,

Option 2. a. Same as above; b. Replace the WP All Import step with some kind of script e.g. “if product category = X, enter google promotion_id Y in field Z” type of approach

I’m also completely open to other suggestions if there is a smarter way to achieve the same outcome. Thanks in advance for any help you can provide.