Extend back to WP 3.5.1?

Any chance this functionality can be extended back to WP 3.5.1? (yes I know it’s only supported to 4.something).

We’re building the client a new site, but there’s a ton of custom php and a separate db with ~20 years of data on the old one which will take some time and effort to port over.

I don’t have full control of the old site’s back end, but I had them insert the ussync_secret_key_1 row into options after installing and activating the plugin. Unfortunately the plugin menu isn’t showing up in admin on the 3.5.1 site, nor is the user verification field in the Users admin.

Any chance that inserting rows in user_meta for all the appropriate user IDs for “ussync_email_verified true” might allow this to work, or is it highly unlikely due to the age of the older WP install?

Thanks for any tips.