The router in question is already terminating a T1 which is routing through to a public LAN block. We're adding a dsl link which is bridging - not routing to a block of addresses. In the absence of an adsl wic card we need to connect the dsl model to a router lan interface. How/Can one configure the router such that traffic to the addresses being bridged across the dsl link can be used by a pix or anything else for that matter, connected to another lan interface on the router?
There are two issues: first is to get the link up, and second is to manage the traffic to new addresses and old addresses.
For the first one you need to know which type of connection is it. If it's PPPoE, then you just need to configure 'pppoe-client' on your ethernet interface and dialer interface with ppp encapsulation.
Second problem is not as easy - for traffic from you to the internet you need to determine whether to send all the traffic regardless their source IP address over the same link, or forward it over T1 or DSL depending of whether it's from IP belonging to your old range (T1) or new one (DSL). If you get T1 and DSL from the same provider, sending all traffic over T1 (pointing there default route) most likely will work. If providers are different chances that you may not have (reliable) connectivity with networks belonging to DSL provider as they probably have anti-spoofing filters on the edge of their network (=not permitting traffic from outside to have source IP belonging to their network range). In this case you have to configure policy-routing on the LAN interface of your router and set next-hop based on the source address of the packets.