1) how much would it cost to "easily feed 2 million additional people"? That food has to come from somewhere, and that's a humongous strategic reserve of perishables if there such quantities just sitting around. They would have to come out of the economy somewhere, in either greater imports or shrunken exports and both could have a devastating impact on an agricultural sector.
2) in what world would a country be given 2-3 years to prepare. Mass movements of people tend to be quick or sudden events, and they're hard to manage. Just look at the hard time the U.S. had in trying to house the 400,000 people made homeless by Hurricane Katrina. And those 400,000 were already living in the U.S., and the factories which made the temporary housing were largely already in existence.
1) It would have almost no impact on the agriculture sector because they'll sell their products at home instead of away : they'll sell it either way. The sector which will get a hit is the import/export companies, but they're less strategic. You're underestimating the immensity of food production (and its remaining potential for growth) in France (and a few other EU countries).
There would be some import needed for the products we don't make but my point was not that it would have no consequences. It's that feeding them wouldn't be leading to lack of food for everyone or enormous additional costly imports.
2) In case you haven't noticed the Syrian civil war started 6 years ago. 3 years ago it was predictable that we would get a massive immigration wave from the area. If France had decided to take a million of them we could have prepared adequately.