You can go through the carrier directly to add any extras such as baggage or seating with no problem on our end. The only exception would be if you added baggage on a no-checked-bag itinerary — in this case, we would not be able to cover you if anything were to happen to your flight or your baggage.
Another action that we’d advise against would be changing your contact details in your reservation. If your trip were to have any schedule changes or delays we wouldn’t know about it or be able to offer you assistance.