Join objects recalculates normals ?

I have an object made out of triangles (alpha mapped grass) and I set the normals for each triangle by hand.
When I want to join multiple grass objects together all normals get re-calculated and I have to set them by hand again.

Is there a way to turn this off ?

As long as i know there is no way, sometimes normals get just wild and i would love to see if there is way