Increasing the handle length only makes it easier to reach the max on the wrench, it does not increase the capacity of the wrench. You need to increase the distance of the wrench to the nut. That would need to be done with an offset adapter, not a normal socket extension and it needs to be in the direction along the long axis of the wrench, from handle to socket. Depending on the type of wrench, that offset adapter might need to specialized to that particular wrench.
Easier would be to get a long breaker bar use a spring scale hooked to the handle and used to turn it. Spring scales are known to be inaccurate so it would need to be calibrated using gravity and a know weight of some other method. Length in feet times pounds give you the torque in ft/lbs.
You might also consider how much force you can pull with both hands on the wrench and your feet against a tire, times the length of the wrench and call it good. Then check to see that they stay tight.
