Sorry to have misunderstood, but are you trying to use After Effects to make a TIFF sequence, or to avoid having to make one?
I don't believe AE reads PPM files, but Photoshop does; you could use Photoshop's Image Processor script to batch convert them. GraphicConverter is another option which might be a bit faster, if you are Mac-based.
Walter Soyka, Principal
Keen Live, Inc.
Presentation, Motion Graphics & Widescreen Design
RenderBreak: A Blog on Innovation in Production