Manufacturer's state that the use of nitrogen in a shock of this type, is to reduce the pressure rise, caused by frictional and compressive heating.
Pure nitrogen is more stable when heated, the pressure only changes very slightly from when the shock is cold, to hot.
You've probably seen tire shops recommending nitrogen over air, for use in passenger tires...same reason race cars and bikes use it in their tires...consistent pressures.
A nitrogen atom is physically larger than an oxygen, carbon or most other atoms in the air we breathe...so it leaks less.
Pure nitrogen isn't flammable, so that's important in certain applications. There also isn't any moisture, compressed air contains enough moisture to corrode internal parts. However, your shock gas is in a bladder.
Hopefully, that explains why you should not add compressed air to nitrogen.
If the pressure is low, you may also notice oil leaks (is there dirt sticking to the shock body?), indicating seal failure. Find a reputable shop to service your suspension.
No, there isn't pressurized gas in your front forks.