I am not clear what you are trying to accomplish.
As I read the documentation, the 826 router was designed to do ADSL over ISDN. If you want to associate several different IP addresses on the WAN interface, I see no reason why you could not use the secondary address capability of IOS. The config on the WAN interface might look something like this:
ip address 192.168.1.1 255.255.255.240
ip address 192.168.2.1 255.255.255.248 secondary.
This associates both addresses with the interface.
But I am not sure why you would want to do this and so wonder if you are trying to accomplish something different.
HTH
Rick
HTH
Rick