There seems to be multiple choices for pmx to pmd conversion available. First would be to just re-save various versions of PageMaker documents (.pmx,.pmd) , which can only be done using the original program
Alternative way to convert pmx to pmd is related to models from MikuMikuDance modeling software. This is most likely used to get different 3D output format for models and can apparently be done only using the original software. You can find tutorials for this on Youtube.
Updated: February 20, 2017