It's a bit unclear - do you have a sequence of files (like animation, drawings, pictures) that are in tiff format? From what you mentioned it seems like there are alpha channels for these images- how are they created? And then the shadows- what format are those?
Be more specific in what you ask, give details and post stills - this will provide better feedback/answers.
Depending of your specifics there can be different answers- for example, you can assign a Matte from a color channel or from the Luminance of another layer... and so on.