How to extend cck field type (add subfields) w/o hacking module

I have a cck type (say "photoset") that is allowed to contain multiple copies of a particular field (say "imagefield"). I would like to extend the information associated with each such field. For example, I might want to associate "photographer credits" with each of the images stored in my "photoset" node.

I am wondering if anyone has thought of a good way to do this without hacking the corresponding module (eg without hacking imagefield.module).

The only thing I can think to do is write custom code that manages a "parallel" database table (eg parallel to the field table managed by imagefield.module) which stores all the extra info. Is this a reasonable way to go? Is there a better way?

Thanks,
Mindy