It is certainly possible for ATF to reclassify certain firearms into Title 2 status by their administrative authority... but only if they fit into an NFA firearm class.
The most recent example was reclassifying street sweeper drum fed shotguns as destructive devices, by saying they did not meet the exclusionary criteria for sporting shotguns with bores over .50"
It could be applied similarly to shotguns which accept large capacity detachable box magazines, or reclassifying bumpfire stock devices as machinegun conversions.
Without creating other classes of NFA firearms, though, which would take an act of congress, they can't just willy-nilly reclassify other existing firearms.
