SQL Server : Transactional Replication Computed Column
You can do this, but keep in mind that your new column will be wiped out if the subscription is ever reinitialized. ie. The DBA at the publisher alters the publication to add a table, corruption, maintenance, etc. and the snapshot needs to be reapplied, any changes you have made to the replicated schema are dropped and recreated from the publication. So any changes you have made will be dropped as well.