I use the PunBB converter to transform XennoBB into phpBB, i make little changes in the conversor and it's works!
But... passwords aren't transformed, users needs restore his or her password via e-mail activation.
I am glad to hear the phpBB convertor worked for you.
As for the passwords, there is a login_mod folder with instructions to make punBB passwords compatible with phpBB. I have no way of knowing if XennoBB uses the punBB password encryption. All you can do is try it. If it doesn't work, then your members will have to use the "I forgot my password" link.