Is it possible to change power convertible top to manual in 1994 Mustang?

The "Power" part of the top on my 66 is only the motor and I'm sure it is on your 94. The cylinders assist the top in going up. If you disconnected the motor, you can operate the top manually. But it sounds like your problem is not in the motor, but with leaking cylinders. You first need to find the source of the leak. Check the hose to make sure it is not leaking, then the connections and make sure they are not leaking. (don't forget the fitting on the motor.) This is a very easy process. If the hose is leaking - replace it. If the fittings are leaking, unscrew then carefully put some Teflon tape on the threads and put them back in - snug them - do not over tighten them. If the Cylinder is the problem - I'd first check a local salvage yard, then a mustang parts catalog, and then ebay. They are not all that expensive, and even a novice could remove and replace the entire hydraulic system in a couple of hours.

You car has to be completely stopped so you cannot open your top while the car is moving. This is a safety feature so you don't try to open it and lose you top while traveling down the highway. Then blow your top at such a crazy mistake.