No, the combustion fan is not speed controlled. We strongly recommend you increase your MIN in order to generate some more draft during these transitional seasons. If you want to continue pushing the limits (make sure you have a working CO detector near by at the very least, and again we do not recommend you burn with such a low draft), you can partially cover the inlet hole on the combustion fan to reduce airflow.
Did I mention you should not burn if your draft is too low to clear the combustion gases properly? Increasing the MIN will increase heat in the stove and lower sections of the chimney and should help generate a better draft when the outdoor temp is warm.