For a shader to be in your collection, you had to have the shader itself in your shader inventory at the time the Collection went live, or any time after that. Simply having the shader on a piece of gear does not count. You can try dismantling gear that has that shader on it to see if it will return the shader, but if it doesn't, then there is currently no way to get the shader, and you will not be able to have it in your collection.