Yes I have this for almos 10 years. NO you cannot vary the power settings at manual mode. It is a pretty simple model only the Z3 have this function. You may try putting soft box, diffuser to help vary the power. Or use the 2 Auto flah mode to help vary the power. It sort of resuces or increases the power slightly, not more than a stop either way.
You will need to try it out and get familiar with the various modes and take note of the differrences.