If your using a current MPFan (V9 & up) it already supports subs. It's all in what you pick on the Parameter page, when your doing your toolpath.
Contour: Pick Depth Cuts and check the Subs box.
If your doing a toolpath Transformation. Same thing. Look for the Subs check box.
You shouldn't have to modify the post to get subs. You might need to modify it for a particular format, but it should be there already.
Mike Mattera
Tips For Manufacturing Training CD's, DVD's for Mastercam, SolidWorks, Inventor, G-Code Training & More
http://www.tipsforcadcam.com