Hello,
While it’s possible to create metafields in bulk using Shopify’s APIs, there’s no easy way to do this via the admin. And this is in case you already have the data for metafields ready.
In case you are looking for an automated way to identify and update the metafields data for all your products - to help with merchandising, filtering, search, etc, we have an app in testing mode for the same and will be happy to have you try it out.