I don’t use SoGo, I use Nextcloud for me & my clients.
Easy enough on Nextcloud to import a single vCard with multiple addresses…
But how to export that. That’s something most clients can’t handle (It seems…)
Mac can!
So export from client via email to myself, I import it into an empty addressbook on my mac.
Then mark all and export it as a single vCard to my desktop.
Then open the Webinterface of Nextcloud and import that vcard to my clients account.
Done!
Left cleaning up the addressbook on my mac for the next client!
Only caveat: needs a mac somewhere… 
My 2 cents
Andy