To be honest, I'd be tempted to keep them both in separate layers. That way, when it comes time to animate, you can rotate the turret independant of the tank.
Still, if you really do want to combine the two, simply enter polygon mode, select all the polys from the turret, cut, select the layer with the tank body and paste. This will put everything on one layer, but still keep the turret geometry separate. If you really do need to physically combine the two, I'd be tempted to do it manually from that point.
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