No. The microarchitectures have some notable similarities and cross-pollinate each other, but they are distinct.
You may be thinking of IBM i (formerly known as AS/400 and i5), which has a completely abstracted instruction set that on modern systems is internally recompiled to Power.
I dunno, but the z-processors and the POWER processors look a lot different even from a floor plan / die shot perspective. The former also clock much higher. Doesn't smell like microcode to me.
