ターミナルのdiskutilコマンドにmergePartitionsってのがあるからそれでやるんだろうけど、
使ったこたあありやせん
mergePartitions [force] format name fromDevice toDevice
Merge two or more partitions on a disk. All data on merged
partitions other than the first will be lost. Data on the
first partition will be lost as well if the force argument is
given.
続きはman diskutilするか、
ちょっと古いけど https://developer.apple.com/legacy/library/documentation/Darwin/Reference/ManPages/man8/diskutil.8.html