Yes, however some of the functionality would no longer be available depending on the placement of your VPN appliance. 

If your network is currently set up similarly to this:

Wall jack  --> VPN appliance --> Wifi router -->devices

You can set up Winston as follows:

Wall jack --> modem  --> Winston --> VPN appliance --> Wifi outer --> Any devices you want connected by Winston

If your network is currently set up like similarly to this:

Wall jack --> modem --> --> Wifi router --> VPN appliance --> devices

You can set up Winston as follows:

Wall jack --> modem --> Winston --> Wifi router --> VPN appliance --> Any devices you want connected by Winston

If there is a VPN on the WAN side of Winston, all of your traffic will be routed through the VPN. You'll not benefit from Winston's IP scrambling (Distributed Privacy Mesh Network).

If there is aVPN on the LAN side of Winston, and that VPN relies on port 443 or port 80, Winston will interfere with that traffic and you'll need to whitelist the VPN domain. However you're essentially circumventing all of Winston's capabilities for the duration that the VPN is active.  

But please note that if you had to set up port forwarding for your original configuration to work, you will need to set up that same port forwarding with Winston. That capability is not currently available, but is on our roadmap, with anticipated completion in a few months from now.