I missed some replies, sorry about that.
I was asking about the addition of bags at the baggage check/ticketing desk and in the end the agent did it, no questions asked. We had five ticketed bags (six including a stroller) in the end but had only paid for two. Of those cabin bags, two were over the carry-on allowance in weight and one was under. We had similar experiences with other carriers lately so we were happy it worked out. They did lose our stroller though, or at least it didn't make it to our final destination with us. Second one I've lost with IAG. That is another issue, though.