Indexes don't have a MAXDOP.
What you can do, when rebuilding an index, is specify the maxdop for that rebuild statement, but it's not persistent, it doesn't get stored anywhere, it's only going to affect that specific rebuild statement. Same as the Sort in TempDB option.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability