I came across some useful links about pnm2ppa on Mac OSX.
You can of course build the source tarball from this site on
OS X, but someone has already done this and nicely packaged pnm2ppa-1.12 with an installer for OS X 10.2.x
I have tested this on my PowerBook G4 with OS X 10.2.6,
and it works fine.
Someone else has also adapted the Networking advice
(from our home page) for printing from Mac OSX
(also OS 9 and earlier, it seems) to a HP PPA printer attached to a Windows computer on the same network
as the Mac (using the postscript emulation method, not pnm2ppa).