Hi Emily, thank you for your response.
Yes, I have tried creating a composite variant and that worked fine. It's just the list of variants that have already been synced from Woocommerce, which need to be turned into composites. I could do this via a CSV import, but that kind of bypasses the integration with Woo, doesn't it? Also, what happens with all those already synced products/variants? Do we have to delete them prior to importing a CSV with the composites?