If memory serves, yes they are available, since they are only characters from other enemy groups. They may need purchasing with tickets as a complete group, but you could probably put one together based on the existing open groups. No idea if they would pop out of the ground, my guess would be nope.
Halloween event (i.e. the treat or treat mobs) are basically zombies from Banished Pantheon.
Now, if you're talking about the zombies from the Zombie Apocalypse event, then they're actually new mobs and not in MA (as of this time).
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ooo good point I forgot about them I was thinking of the trick or treat enemies.
Trick or Treat mobs are -mostly- based off existing mobs:
Coven
The Coven is made up of witches also seen as the Cabal enemy group found in Croatoa. Note that this group is the most visually different from their normal in-game spawns of any of the Halloween mobs. The Cabal is primarily colored in shades of grey and black, while the Halloween Coven is much more colorful.
Fir Bolg
Fir Bolg can also be found in Croatoa at the present. Note that there is no badge for defeating them.
Spirits
Spectral Demons from the Circle of Thorns were used to create the Spirits faction.
Vampires
5th Column vampyri were the models used to make up the Vampires enemy group for the event.
Werewolves
5th Column nightwolves and warwolves were the models used to make up the Werewolves faction for the event.
Zombies
The zombies are Banished Pantheon soldiers.
The Zombie Invasion event mobs are unique, and presently not available in the MA.
As far as i know they are GM status critters and not available for MA missions. It would be cool to use them on that Mayhem Hospital or Graveyard map tho". 
yep it was the apocalypse zombies I was looking for ( i mentioned halloween because that was when they first turned up as far as I can remember... or was it valentines?).
anyway, thanks for that 
here's hoping they introduce them as lvl 1-54 mobs in the near future.
The apocalypse zombies use the invasion code for scaling, which makes them unlikely candidates for the MA. I think that they all have an actual level of 25.
I really should do something about this signature.
I would really like the invasion Zombies, the EB's are looking totally awesome.
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The apocalypse zombies use the invasion code for scaling, which makes them unlikely candidates for the MA. I think that they all have an actual level of 25.
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Wouldnt it be minimum of 27 since they drop level 30 recipes too?
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Could well be 30, I was just going on memory. what I know for sure is they are all the same level.
I really should do something about this signature.
anyone know if the Halloween event mobs are available in the MA ? Do they need to be unlocked ? do they pop up out of the ground ?