Is it possible to use Pi's output signal as Arduino's input signal?
This way is fine: the Pi is a 3.3V based board, so connecting one of the GPIO pin from the pi to an Arduino will not damage it. Furthermore, the 3.3V are sufficient to be considered as HIGH even on a 5v Arduino like the UNO.

The other way around: connecting the output of a 5V Arduino to the input of a pi, will very likely damage the raspberry Pi! 

So beware of your Pinmode before doing such a connection!