I do not use or have experience working with soft bodies, so I cannot provide you with an immediate solution. Hopefully someone will respond with a solution.
Assuming you may not have considered, the solution may be available at NVidia or another game engine forum which provides PhysX functionality, such as unity.
You are more likely find the answer to this kind of issue elsewhere because the vast majority of PhysX users are not here. Although I am not suggesting that you will not find some PhysX technical guidance here.