Hi,
There is an option in the OpenVPN configuration to specify the client's private network address in the 'Remote network IP address/netmask' fields:
Replace the highlighted data with the client's actual private network information and you should be able to gain access to the client's networks over the OpenVPN connection. If you also wish to grant access to the server's private network for the client, you can specify the same thing in the client's configuration:
Once again, replace the data with the actual network information (your server's private network).
Good luck,
DM