yes... all you need is a 3 way light switch... u can get one virtually any hardware/big box construction supply outlet... just wire the incoming voltage (line) to the black screw, and the remaining gold screws, one goes to the high, and one goes to the low. however doing it like that will take away the ability to shut the fan
off if you need to, without adding another simple single pole (on/off) switch... if you wanted to be able to shut the fan off, simply intercept the incoming voltage with a single pole switch.
EDIT-
proofreading i noticed that i had misread the original question... beaver heater had asked to be able to do this automatically...
yes there is, there is always a way.... what you need, is one of these-
click image for ordering info cost = $16.83 usd plus S&H
a single pole, double throw, relay (SPDT) with a 120v coil.
the concept is pretty simple.... THE COIL will be controlled by the same circuit that controls your lights.... i know you have a timer there somewhere... THE COIL connects to the timer output.. just like another light... now on the circuit that runs THE FAN, the hot wire only, connects to the relay COMMON. the NORMALLY CLOSED wire on the relay will connect to the brown LOW SPEED on the fan. the NORMALLY OPEN wire will connect to the black HIGH SPEED on the fan... and thats it. while the relay is in the normal, deengerized position, it provides power to the fan low speed while the lights are off. when the timer kicks the lights on, it will also energize the relay, which will cause it to switch power to the high speed pole...... hope that helps!