How can I convert the current from a fan to the microphone headset input on an iphone?
Instead of trying to input analog levels, you may have more luck if you use your input to control the frequency of an audio oscillator.  You would then measure the frequency in software on the phone.

But you might not even need to do that - if not equipped with filter capacitors, your generator already may have an audio-frequency variation proportionate to speed in its output due to the poles and brushes.  If you had a brushless device which output A/C, this would work even better, but you might be able to do something by A/C coupling the output of your current device with a series capacitor chosen to pass only the brush noise.

You may also want to think about a protection circuit for the phone, such as some back to back diodes.