For info we had a lot of discussion a while back about the PSR 2000 / 2100 / 3000 microphone input; as I recall we concluded it is unbalanced with no phantom power capability. I assume this is still the case for the s900.

Certainly these keyboards can put reverb or other effects onto the microphone signal, and there is enough gain to cope with a low-level microphone, but at high gain you may well hear noise from the pre-amp. Use of a small mixer to bring the microphone signal up to line level would give a better result.