Accessing extra channels in EXR with glsl


  I am attempting to overlay some channels stored in an EXR on top of the RGBA using a glsl script.  But, my problem is I don't know how to access those other channels in the EXR as the inputImage returns a vec4 (RGBA).  My EXR has 6 channels in it and I want to do something like this p-code:

vec4 main (const in inputImage in0)
vec4 P_rgba = in0();

vec4 P_exr1 = in0("extraChannel1")

vec4 P_exr2 = in0("extraChannel2")

return vec4(P_rgba.rgb + P_exr1.rgb + P_exr2.rgb , P_rgba.a);

Is this possible using glsl?