Thank you for your useless advice!
Openvpn is already integrated into the image! You may not have read my question carefully. I asked about network-manager. In versions prior to 4.7 inclusive have been able to use network-manager to create client VPN connections. It was also possible to import saved VPN connection settings. In version 5.1, these features remain for PPTP and cisco anyconnect only. Installing the network-manager-openvpn-gnome and network-manager-openvpn packages does not yield results.
There is option... Backbox 5.1