This is in our product roadmap and will be delivered to you as a firmware update later in the year.
The slightly longer answer is that our product is a linux router (OpenWRT based) which is configured to use OpenVPN in a specific way. If you're knowledgeable enough in Linux and networking, you should be able to change the configurations to work with your own VPN. But we're not going to be able to provide you with instructions or support for this.
If you go for option two, have fun. The InvizBox Go is a nice piece of hardware to hack (if you go too far, we're here)!