Site Categories - SQL to Add Categories for Existing Sites?

I have approx 1,000 users who have created sites on a multisite platform. They have categorized their sites in BuddyPress, extra profile fields. However, this is not user-friendly. I just found "Site Categories" and added it. Now, I need to update the existing sites with their chosen categories.

I've been pulling my hair out over here trying to figure out where all of the data is located and how to get at it.

I've found the following:
> Categories in wp_terms
> Categories selected by Site in Site Categories - wp_signups (meta column)

I've tried adding the meta code, but it didn't seem to work. I could really use some step-by-step instructions on how to add the correct categories to the existing sites via the database.