When you are about to add the fragment you can use the FragmentTransaction class to set the animation.
So something like this...
FragmentManager fm = activity.getSupportFragmentManager();
FragmentTransaction ft = fm.beginTransaction();
MyFragment fragment = new MyFragment();
ft.add(android.R.id.content,fragment ,TAG);
ft.setTransition(FragmentTransaction.TRANSIT_ENTER_MASK);
ft.addToBackStack(null);
ft.commit();
This should lay your current fragment on top of anything else without removing anything. The "setTransition" animation will animate the fragment entrance but I would make an attempt to use "setCustomAnimations" to get full control of how the fragment will be animated.