Presuming you're talking about the "RRD fan extender", unfortunately it's meant to occupy the same spots as where the servos are at. Who knows why their lousy design occupies all 4 servo slots. You can make a intermediary board that splits out the pins so that the extender doesn't unnecessarily use two additional servo slots, or you could use jumper wires so that only the necessary pins are used. You could also use a different fan controller that's controlled from PWM pins D44 and D45 which are unused under standard configurations.
