I assume you are using a layer 2 6509...with 7507 being the external router. Since you have added a new subnet, as secondary, ospf wont send hellos on stub networks such as the secondary network. You can create two subinterfaces, on the fast ethernet, interface, and also create two vlans, one each for each subnet, and make the 7507 route between these vlans. You need to enable ISL trunking or dot1q between the 6500 and 7500. You dont need port channel for this. You can add as many subinterfaces as you want (practically speaking, limited by memory of router). Once this is over, you can add the new neighbor to the newly created vlan, and things would be good to go.
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus