I cannot find a way to get SUMA to look at a different repository, at least not directly from the SMIT menus. "Maybe" if you create a cron job it will go to a local repository.
However, imho (in my humble opinion) the whole point of SUMA is that you create your local update respority that can be used as nfs mounts by local systems. I have a AIX 5.3, 6.1 and 7.1 system, each with there own suma settings.
I consider myself as having local repositories that I fill using SUMA and save me the trouble of going to fix central to look for what is needed. Or - maybe I am not understanding you correctly. You can configure SUMA to compare with a directory so that it only downloads what is missing. Is this what you meant by repository perhaps?