Using already defined expressions in Python

I have defined a couple parameters (which are accessible through Expressions window) to run positional parameter sweeps by using body models. I am using parameters dx, dy, and dz for definitions of translations along with x-, y-, and z-axes respectively. However, I cannot modify these parameters from python interface. I need to reach these variables and modify them.
Is there a way to modify parameters that are already defined in Expressions window, by using python API?
Thank you.