Download useMatte v1.4 (2kb RAR file)
It works very simply — it has two inputs, one for the image and one for the matte you wish to assign to that image. Check Mult if you want to multiply your image by your new matte (for additive compositing), or uncheck it for subtractive. Float folk check Clamp to ensure that no out-of-range values get used in the matte. The only other control is a channel pop-up, for selecting which channel to use as the matte.
ProLost — Teaching the World How to Use Fusion Wrong™.