pptp connection change external IP on a FIXED External IP Co

Hi.. i am using a SME 5.5 u2 with fixed external IP, on a Public Gateway & Server Mode.

I make one additional conecction for make access to PPtP VPN server( not SME)

I download and install pptp-linux rpm and CAN obtain a pptp conection that create a new network interface: ppp0. Later that interface ppp0 is UP, the new ip obtain is assigned to red interface o external IP.

Any body now what script i need modify to:

1. Not modify external IP address
2. Routing packet from my internal lan to the external interface for internet access and routing packets from my lan to vpn server connected by de new pptp conecction for ip range of vpn(pptp) server and addition private networks ?

Thank's and sorry by the english

..

Luis