MySQL Replication, Can i add a trigger to the SLAVE db? Is it possible to do so? The trigger will insert some rows into a unique table that exists only on the slave.
"Is it possible to do so? The trigger will insert some rows into a unique table that exists only on the slave."

yes, you can

A lot of warehousing architectures are based on the slave only.

The title of your question is wrong. Change replica DB with SLAVE ONLY