Kennon's post about covers the way to change the pdt file used by a scenario, for those of you who aren't aware of it there is a webpage that provides information of the various files used in the HPS and Tiller Games and how to modify them.
If you're interested check out the ACWCO Engineering Department -
http://home.comcast.net/~acwco_engineering/You'll find a section describing the various lines of the pdt and what they control so you can modify the values you wish to change, this is current to the Overland game changes.
There are also sections covering the .oob and .map files.
And you'll also find a section with an intro to using the Scenario Editor that should assist you in opening a stock scenario (.scn) file, saving it and adding comments about your changes. This section also includes another example of how to make the changes for using a non stock pdt file as described by Gen. Whitehead.