Workerids are unique, you can't register my registered workerid to your pool account. But you can mine for someone else's worker, which I think may be a huge problem now when the threads are limited, because someone could just kick out of the pool anyone not anonymous by simply mining with a few more threads on their workerid and cause them to go over the limit which will have them revert to solo mining.

This is why I just changed to anonymous and changed my workerids.