We exported sample human object(.obj) and imported into Blender. Using blender script, how can I modify the face parameters (oval/round face etc) dynamically ?
You can use MakeTarget to load and apply MakeHuman targets (for example "face round"). This can be done via MakeTarget's UI panel.
MakeTarget also provides a basic programming interface for loading and applying targets. This is used by, for example, MakeClothes.
Unfortunately these features are largely undocumented, but you can try to read the MakeTarget source code to figure out how it is done. See the MakeHuman bitbucket repository at https://bitbucket.org/MakeHuman/makehuman/src/6cee93d5c66ad29d37836daaa3fec6a322751219/blendertools/maketarget/?at=default
If you need to discuss these things or request more help, you would probably be best off with asking about approaches in the scripting subforum in the makehumancommunity forums at http://www.makehumancommunity.org/forum/