If an employer concedes arbitrability of a question, an arbitrator reviewing a collective agreement that prohibits changes in arbitration may reform the terms of the agreement for mutual mistake, subject to rational basis review; such reformation does not offend public policy.
Dissent: As the arbitrator's authority arises from the collective agreement, the no-add clause stripped the arbitrator of jurisdiction sufficient to amend.
http://cdn.ca9.uscourts.gov/datastore/opinions/2018/06/19/16-16363.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.