There would be two ways to perform PDF to AFP transform.
The easiest way is to convert the PDF to a series of TIFF images and rewrite the AFP stream to include them as IOCA images. If they are two-color fax G4 compressed, they can be converted to IOCA group 10 without much trouble.
The second way is to convert the PDF to AFP and rewrite the AFP stream to include it. If the PDF doesn't have embedded fonts then you shouldn't have too much trouble mapping the fonts to the AFP ones.
The first way generates full-page images, i.e. rasterized AFP pages, text is not searchable, graphics is not vector based. It may not look quite right at first impression.
PDF to AFP Converter (PDF2AFP) follows the second way and generates full-text searchable AFP files with vector based graphics.