It is quite feasible and available on other PT-6 family engines. The PT-6C engine is available with FADEC but appears to be more for rotorcraft. That being said its fundamentally a similar engine and the systems could be migrated to the common PT-6A.
The question of:
I am wondering is there any necessity to equip this engine with FADEC
This is somewhat of an opinion based question depending on what you personally feel is necessary. FADEC allows a pilot to run an engine at optimal performance all the time without needing to babysit the various engine parameters and settings. Some see this as a great benefit (taking something off your plate in a demanding airplane) while the more classically minded prefer to have finer control and operate their engines in various ways that may be considered less than optimal but appropriate for the mission.
The cost of FADEC is putting the engine control in the hands of a computer, which many are slowly becoming comfortable with. One thing to consider on smaller planes like the M600 is backup power systems. Large planes have the luxury of things like an APU and a RAT as well as lots of alternators and battery banks to mitigate power failures across 4 discrete power sources. The M600 has only its alternator and batteries as a backup so in the event of a power loss you have less to fall back on and may lose the engine.
whether the cost/benefit ratio is justifiable
Without knowing P&W's financials its hard to say. They are clearly selling lots of PT-6's and many might say theres no reason to change something that works. Since they clearly have a FADEC system for the PT-6C I would imagine a great deal of the work has been done and the system could be migrated to the PT-6A at least in part but you would need someone from PW to confirm that assumption.
Just a bit of a history note the PT-6A is a fairly old engine and has been around since the early 70's a time long before FADEC was small enough for something like the M600. On the same note the PA-46 first flew in '79 a bit before FADEC was viable on a small scale. There is some reality to keeping up a certified config once its certified.