That's fairly easy. Airports are closed right now, the old border posts, even though they have been un-manned for quite a while (but not everywhere, as I found out to my surprise a few months ago, a so called 'spot check' was set up on one of the old border posts) can be easily re-established using military personel.

It's a lot easier to re-establish something that was there before than it is to do it from scratch and this is more of a logistical problem (and a personel one) than one of technicalities about whether or not such a thing can be done. It's not like France didn't have a functioning border patrol a couple of years ago.