Can a 15 amp electrical switch be used to replace a push button switch for garage door opener?
A switch can be used for any voltage from it's rating or less, voltage wise. Thing is, a garage door opener button is momentary, where a wall switch is maintained. There is the possibility of damage if you leave a switch in the on position too long in an application like this. I wouldn't do it. 

The problem you have is that someone ran this low-voltage wire in the box with the line voltage wiring. This assumes the wiring is something like thermostat wire or what came with the opener. 

I would remove the wiring from that box and mount a regular opener button to the wall.  

And @gregmac makes a good point. If this opener is newer, as in maybe ten years or less, then a regular switch may not work, or at least it will not give you all the functions that the opener offers.