Well you simply need to drop the voltage from 9v to 3.7. You can more acuratly do it with a regulator, so the voltage doesn't dip until the battery is nearly dead. 9v batteries arn't terribly capacious though, I hope you know.
If the battery from your old phone is close to the same size as the new phone, you could adapt it to work even easier, assuming they're both lithium ion batteries.
[This message has been edited by Jax184 (edited 01-26-2007).]