Shouldn't be a problem, providing you don't create unique indexes that don't exist in the publisher.
If the subscription is reinitialised, you'll have to recreate the indexes.
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
We walk in the dark places no others will enter
We stand on the bridge and no one may pass