With power off, is it possible to maintain constant altitude for some period of time, until the stall speed is reached?
Yes, assuming your are flying level with the engine running and then you turn the engine off, the following will happen:


Without power from the engine, the drag will cause the airspeed to decrease.
Lower airspeed at the same angle of attack implies less lift. Since you want to maintain altitude, lift needs to stay the same (your weight has not changed). Thus, you have to increase pitch.
Eventually, you will reach the critical angle of attack and flow separation will occur, i.e. you stall. The airspeed at which this happens is called the 1-G stall speed, although one can stall any speed.