How to migrate from a MySQL master-slave setup to a master-master setup without downtime
On a basic level, yes. Just set up the current Master like you would any normal slave database.

However, there are some things to consider first.

Is the second slave set to READ-ONLY, as you want to avoid writing to both masters at once.

You probably want to set auto_increment_increment=2 and auto_increment_offset=1 on the first master, and auto_increment_increment=2 and auto_increment_offset=2 on the second master.

This prevents the two masters from creating the same auto_increment primary keys.

You also want to make sure you have no replication filters (binlog-do-db, replicate-do-db etc) set up as they can cause problems too).