If you want to move an external from MaxMSP to MaxMSP/Pd, I developed another approach which is far less elegant than Thomas Grill's flext, but may be easier when you are starting with an existing MaxMSP code base. The basic idea is to use #ifdefs at the few places where MaxMSP and Pd code diverge, and a single header to wrap the different headers used for the two programs. The result is a single code file that will compile into either Pd or MaxMSP. You can see examples of this here:
which by the way corresponds to flext version 0.1. (might still be found in the archives!)
This is the best approach for external objects that only make use of basic functionality of the underlying system.
greetings,
Thomas