You can check if the N73 battery has the same voltage (3,7V) as the N900 battery and the same pins/contacts (+,T,-). If it does, just connect the N73 battery comparment contacts/pins with wires (avoiding short circuits) to the N900 battery according the order of the +,T and -. Then just plug the N73 to its own charger and voila. the charging circuitry monitors the charge level by the voltage, so it doesnt matter that the batteries are of different capacity as long as the nominal voltage is same.