Someone made an adapter that uses a Tesla-compatible socket and a J1772 plug. He has used it to charge a BMW i3 from a HPWC:
BMW I3 Charging on Tesla HPWC
This works because the HPWC protocol is the same as J1772. But it will not work on a Supercharger because of the Tesla-specific communication.
While there has been a lot of speculation about how Superchargers can prevent unauthorized vehicles from charging, I don't think the Superchargers are on a live network and authenticate the car via a central server. Many Superchargers are in the middle of nowhere and live communication to a central server would be difficult or impossible.
Instead, I speculate there are two separate authentication mechanisms:
1. The car does not allow supercharging unless that feature has been enabled in the car. This is how the 40s and 60s that aren't supercharger-enabled work. They simply won't tell a supercharger to begin charging.
2. I'm betting that the communication packet that the car sends to the supercharger contains:
a. The car's VIN
b. Date and time (to prevent replay attacks)
c. Whether the car is authorized to supercharge or not.
d. Maximum power the car can take.
e. Amount of power the car is requesting.
f. This entire packet is digitally signed with a private key embedded in the car's firmware, and possibly with a car-specific key.
The supercharger would then validate the digital signature using the other half of the signing key.
So even if you make an adapter and reverse-engineer the proper communication protocol, there is no way for you to sign this packet without the private key, so you'll never get the supercharger to supply power to you.
This would also allow Tesla to revoke permission to supercharge from another manufacturer who bought into the supercharger network if that manufacturer's car firmware was compromised and the private key leaked. You just revoke that key from the supercharger firmware.