It would have to be VERY compelling evidence for a judge to allow a jury to hear it. Even then, one crime doesn't excuse another, so it's relevance to their guilt or innocence is suspect, if also possibly beneficial at sentencing.
There are PLENTY of cases where FBI agents broke wiretapping laws on some very bad people who I'm sure that were guilty of some very bad things, but that doesn't inoculate the FBI agents from the crime of an illegal wiretap.
I guess what I’m saying is regardless of the legalities of impersonating phone company employees, maybe O’Keefe and friends simply wanted to appear to be bugging the phones. For what reason, I’ve no idea but I think there’s way more to this than meets the eye.