We can sort of do this already.
@OneirosTheWriter has indicated that we have the ability to "mothball" ships instead of scrapping them, so theoretically we could build, say, Centaurs, and then immediately mothball them until needed. The difficulty being that these aren't ships that could instantly be brought on line. Remember that the custom is for a crew to shake out on a new ship for a full year before it's actually brought into service. While a state of emergency would no doubt bring that down, it would likely still require some months (and crew) to bring them into operation.
As for "purpose-built warships" we can do that too if we want... but you've seen the discussion regarding how much research time it takes for a new ship design. Kind of hard to justify doing that for a ship we don't even plan on using regularly and then continue to pour resources into ships that we aren't going to use regularly. We would only want to do it if we were so far ahead on ships that we want that we've hit our militarization cap and already have the ideal active fleet mix we want... but that's still a ways away.
