Maybe some sort of sandbox mode? Where you don't have to worry about staying alive. Like either spawning individual infected or some sort of ceiling to prevent floating away, and of course invincibility.
Two things.
If you want some configuration to set a point of despawn (usually for low end pc), you can set it on the configuration menu (between 30 - 200 meters of celling).
By other hand, if you want something like "keep the enemies coming, until i blow!", you can enter in Hard mode, fight until the round 8 or 10, and don't blow up the blimp.
While the blimp is still alive, the enemies won't stop coming.
Note 1: those aren't the same thing as an "infinite mode", or a "sandbox mode", but they are closer to something like that.
Note 2: The "hard mode" was not tested before release, so... yes, it's quite unbalanced in terms of resources needed... in round 10, you can even see the frames.