Viscosity is my OpenVPN client of choice. Recently I wanted to be able to route through my office to connect to some clients that have ip rules limiting the source of the connection. It's easily done:
For each IP you wish to route through the VPN:
1. In the Routing section click the + button to add a route. An Add Route dialog will pop up.
2. Add the IP address of the host you're trying to connect to in Destination.
3. Add the bitmask 255.255.255.255.
4. For the Gateway, select VPN Gateway.
5. Click Add.
Once you're all done, click Save.
Next time you connect traffic to the specified host should be routed through your VPN connection.
Check your work using: