PPTP forward port per user

Hi, i would like to be able to forward an internal ports of users through the VPN. The idea is that a user picks 2-3 ports (or maybe just one) tcp port out of a given port-pool, and when he connects to the VPN, this portforwarding is established for him. Any idea how to do this? I'm using pptpd with freeradius + mysql. thanks for a hint. cheers, --sascha