Yes. Many have. Take a look at this guy on YouTube:
https://www.youtube.com/watch?v=Ztejn3YYbvw&list=PL5Nz1FJp-6dtcfy52ArsE6-EyGCAKRlS1
There are several ways to do it.
The easiest way is to make an auxiliary battery that has the same voltage as the Leaf's traction battery (96 Li ion in...