yes, can be done anytime after booking., and even at check in. You’ll need to call in though - can only be done online if booking with the adults.

Also note, IME, that it’s a good idea to confirm the adult traveler info after the agent books the lap infant - maybe I’m confusing all my travels to Canada/international vs. domestic, but the system has always lost the adult secure flight info (birthdate/Trusted traveler number) after they’ve added the infant to the booking. so check to make sure, so you can give that info to the agent if that happens.

for domestic, this is easy - as there’s no fare/taxes to worry about. For international, where the 10% fare for a lap infant is charged, its done based on the fare at time you book the child (IIRC), so there is an incentive to do it at the same time.

also, when selecting seats, note there are some restrictions with a lap infant on some aircraft. Obviously, you can’t sit in an exit row, but beyond that, the Express aircraft have some interesting restrictions. On a CRJ (-200 and -700 for sure, and I’m assuming it’s the same on the -550), you can only sit on the AB side of the plane, and on an E45, there are only 4 rows that can accommodate a lap infant. This is all did to these being the only places where there is an extra oxygen mask. Better to know this and be able to select the proper seats in advance - I learned the hard way with our first child, and had to be moved on a couple of those express flights by the GA or FA.