Para 15.6 execute o seguinte como root:
zypper addrepo https://download.opensuse.org/repositories/server:Kolab:16.2/15.6/server:Kolab:16.2.repo
zypper refresh
zypper install roundcubemail-plugins-kolab
Para 15.5 execute o seguinte como root:
zypper addrepo https://download.opensuse.org/repositories/server:Kolab:16.2/15.5/server:Kolab:16.2.repo
zypper refresh
zypper install roundcubemail-plugins-kolab
Para 15.4 execute o seguinte como root:
zypper addrepo https://download.opensuse.org/repositories/server:Kolab:16.2/15.4/server:Kolab:16.2.repo
zypper refresh
zypper install roundcubemail-plugins-kolab