Adding a CCK field with Rules
Does it need to be a user reference?

Assuming that you are not using a global flag (then you wouldn't have an issue with multiple users), that information should already be stored in the flag tables. You should be able to show that information with a view (which you can attach to the node with http://drupal.org/project/views_attach). Or if that doesn't work, do a custom query in a hook_node_view() implementation in a custom module.

And if it has to be a user reference for some reason, an unlimited multi-value user reference should be easier to handle with Rules than a unlimited number of user reference fields. CCK (nor Fields, for that matter) is just not designed for that. And you can only add fields to content types, not single nodes.