You could use something like a foot operated air pump or even a bicycle pump, I suppose.
But do you really want to use "air" to displace beer? I mean, it's fairly anathematic to mix beer and air.
While I'd love to understand why one would actually want to do something like this, it'd make more sense to use bottled CO2 or an inert gas like nitrogen or argon with the appropriate regulator...
Cheers!