To answer your original question, no you can't reassign an airplane that you bought from the Used Market that has not delivered yet. The same goes for planes you order new that are within 3 weeks of delivery.
If you have any A321 at your HQ, what you could do if you do find yourself about to forfeit slots at a base would be to move the schedule of one of your existing A321 to the plane awaiting delivery and then rebase that plane so you can fly the new route immediately. But only do this if the plane you just bought is less than 14 days from delivery (so wait a couple of game days before transferring the schedule).