Thanks to "Phill" for the example code, but be observant of his note:
NOTE: As the package is being modified programmatically, you will lose any text annotations and layout previously applied. Unfortunately there is no way to modify the package outside the DTS Designer and maintain these items.
This means that script will save the DTS package with the "Auto Layout" option applied. If you have spent any time arranging package steps in the designer, it will be lost.