Yes you can do that. It's done by making the cable between the two boxes a trunk cable that is tagged with VLANs. This is called a one-armed router system since packets enter the Pi, get routed, and exit on the same port (but with different VLANs, so they two networks are logically isolated from each other.) Inside the 8970, one VLAN on the Pi's trunk port would bridge to the DSL modem, and the other one would hardware switch to the other Ethernet ports and software bridge to wifi.
The way this is configured is completely different whether it is a swconfig or a DSA kernel, but it is possible with either one.