We have just released an update to the User Sync for Klaviyo plugin.
This new version now introduces hooks, filters and actions so developers can extend the plugin and add custom information that will be sent from WordPress to Klaviyo.
You can find all the information on this here
There have also been some minor bug fixes and tweaks to the plugin in this latest release. Full release code can be found on github and the plugin is now updated on the WordPress.org Repo