hey man, if you want to do pretty decent re-lighting in shake and you have shake4 go to www.pocket-rocket-fx.com, go to the tools page and download the QuickLights plugin and the Rotote Normals plug-in's. Once you have re-lit them you can just us and iAdd to add the new light on top.
as for you're question, are you doing like
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normals
/ | \
rrr ggg bbb
then doing your mask stuff?
if you have separate channels just iAdd them together to create the final color image..(RGB is an additive colour space)
cheers,
Pat