yes, this is possible with following plugin: https://github.com/sblaisot/automatic_addressbook
You may also check following link for installing plugins: http://docs.nethserver.org/en/v7/webmail.html#plugins
I like that feature too so I just tested it and it worked so far:
tar -xf 0.4.3.tar.gz
mv automatic_addressbook-0.4.3 /usr/share/roundcubemail/plugins/automatic_addressbook
mysql roundcubemail < /usr/share/roundcubemail/plugins/automatic_addressbook/SQL/mysql.initial.sql
config setprop roundcubemail PluginsList "managesieve,markasjunk,automatic_addressbook"
After sending an email, the address is saved to the “Automatically Collected” address book: