It depends on how soft and edge you want. You can use lengths of rolled up tape just inside your mask edges to lift the edge of the mask a bit. With som experimation on air-pressure and distance form painted surface, you might be able to get the right effect. I have used that method prior to having a controllable airbrush, and it worked somewhat. The effect wasn’t so much that of a soft transition as that of simply taking a bit of the starkness from it.
What kind of airbrush do you have? Even a very simple single action like a Paasche H can easily do fine enough lines to allow you to do free-hand camo, but like everything with airbrushing, it is a matter of practice and experimentation.