Generally no, you can not Push because to do so you usually need to be taking an action and you cannot do that if it's not your Phase (barring a Held Action like Greywind said). That said, it is not explicitly banned and a GM could definitely allow you to Push in a STR vs STR check initiated against you or even Push a Defensive Power you Abort to (or maybe just Push a Defensive Power in general if it was already active) as long as it fits the criteria listed. "Pushing is only for crucial, heroic, or life-saving situations", "Pushing is only for heroes and important NPCs", you declare it before any roll, you make your EGO check (if it's a Heroic game), et cetera.