how to increase chances to get in to chongfu primary? eg volunteer?

There are many ways -Volunteer - Alumni - Distance (less then 1km) These are the additional factors for getting into a popular primary school.

just to give you a heads up, not all schools accept volunteers you may want to call the school and check if they accept volunteers.

I know of students who transferred into that sch using their p2 results ... w/o thecparents doing anything prior to that

Why not call the school to check. It's very competitive now due to many new estate near the school.