How to add another system (stave) under the first one that has been edited..

Press I and add a staff or instrument

A system is different form a staff. If you want another staff - more notes to be played at the same time as what you already have - then indeed Edit / Instruments allows that. If you truly want another system, though - more measures to be played after what you already have - then use Add / Measures to add more measures. They will flow to a new system once there is no more room on the current last system.