The short answer? You don't.
It's not so much a question of whether or not DriveWorks could do it. DriveWorks essentially just uses the SolidWorks API to modify parameters, and there is a call stack that can be used in API to modify global variables. If you're using DriveWorks Pro, an enterprising API enthusiast could probably connect the dots.
So why not do it that way? Why doesn't DriveWorks just allow you to modify SW global variables without an API? Their stated reason in their help files is "maintenance purposes", and I'm inclined to agree. Documenting your logic behind your design is one of those situations where you'd rather put your eggs in one basket, so it's easier when it comes time to change.
Or, how to capture the length of an extrude, so I can update it with data entered on the form? Seems like this should be easy...