Can i plug my stage piano into my hi fi amp?

The amp might be okay (line input equivalent would be Tuner, Tape, Aux) but I'd worry about hifi speakers as the piano signal will have more attack and dynamic range than highly compressed domestic audio sources.

you might want to invest in PA speakers.

all instruments, playing LIVE sources have huge dynamic range, warmth and thick textures, connecting HIFI , stereo, HT speakers will just end up the speakers with blown drivers..

what kind of connector would you use insturment jack to --> ???