Hi
I did the same thing a few years ago, you will have the find the closest next up version of your MySQL to MariaDB. Some jumps are simply too big fore teh upgradee scripts. Mine was a 5.1MySQL going to a 5.5 MariaDB. We moved to 10 about 1 year later. That was all around the time 10 was coming out. You can add MariaDB as a slave to any of your MySQL servers and then do a switch to it as the master. I would recommend that you make it a direct slave to your existing master
Steps from memory
Make a slave as 5.1
remove 5.1 software
Install MariaDB 5.5
Run upgrade scripts to MariaDB
Switch to master
Obviously somewhat more work around when and how you manage to point your app to the database?
I would suggest you run whichever version of MariaDB in your dev and test but not necessarily UAT environments as part of doing this and build a master-slave unit with your current MySQL in say test and then do the switchovers a few times there to confirm your whole process.
Cheers
Peter