MONyog calculates the volume usage by executing ‘df’ command on the target machine. MONyog gets the information about data directory(and the volume where it resides) from 3 different system variables: basedir, tmpdir and innodb_data_home_dir.
MONyog.System.Disk.sys_disk_free_innodb returns the amount of free space which is left on the volume where InnoDB data resides.
You can use the other MOMs to create a new monitor or change the behaviour of an existing monitor but you cannot add any extra MOM.