IPTables interface prerouting

I have built a minimal firewall (on a iPentium 100) using kernel
2.4.20, have a quite good rule set running.

I tried to find a way to connect a notebook to my lan, without needing
to buy a pcmcia(16bit) NIC for this ooold fashioned portable. I tested
connecting it to a linux pc (with a NIC of its own) and linking both
via a nullmodem cable using pppd, then using NAT to bring all packets
from interface pppX (the notebook's) into my LAN. Worked fine.
But:
I want to use my minimal router instead of this second PC. AND: I
don't want to change all of my rules just to add support for another
ppp interface. What I mean to say is: (How ) Is it possible to
configure Iptables that way, that during its prerouting methods it
routes the packets from ppp1 to eth1 (my ethernet interace on the
firewall) before continuing processing those, so that I only have to
add some rules instead of modifying dozens of them?

Greets
Marcus