Hi @curunir,
by default the mail addresses in the roundcube LDAP addressbook are bound to NethServer users AFAIK. One would have to reconfigure openldap (new addressbook ou) and roundcube (ldap filter settings) for allowing that. I don’t know if it’s possible to write to LDAP via roundcube, so you may use phpldapadmin in kamikaze mode to add mail address entries.
I actually have no experience doing this but when I find some time, I’ll try it.
Found some links:
https://www.brennan.id.au/20-Shared_Address_Book_LDAP.html
Writing to LDAP:
https://community.nethserver.org/t/profile-photo-in-nextcloud/7783/16