Hi at all,
i found a solution for my issue.
I create in zentyal a new vpn-user.
I take the certificates from nethserver out of the path /var/lib/nethserver/openvpn-tunnels/keys
name-of-tunnel.crt (rename to cert.crt)
name-of-tunnel.key (rename to privateKey.crt)
this files i choose for setting up a new vpn-user in zentyal.
In Passwort it doesnt matter wich you choose. It is not required for zentyal to nethserver vpn but has to be filled in.
For me it was also important to mark TUN.
Save and activate.
Now i had the connection between both.