Hi @serpico, welcome to the forums.
The imapsync is meant as a migration tool for email accounts from one provider to the next and not as a live tool used consonantly to update synced accounts.
Nethserver has a module within the email section called Connectors that will enable you to retrieve email from external sources like I suspect you are after.
This last section I think might be up to how you want to handle your setup. If mario@ is an active account someone uses to send & receive emails you could consider making a shared mailbox/folder, creating auto-forwarding rules for stefano@ & manuela@ or having multiple accounts mapped to your email endpoint in Thunderbird/Outlook/SOGo/Webtop …