If you're travelling on KLM, it doesn't matter how you booked the ticket; you can just enter the PNR (the 6-character reference number) and the surname on the KLM website, where you can manage your booking. This allows you to select seats (for a fee) or add baggage.

If the agent that sold you the ticket is withholding the PNR - though it is more than likely indicated somewhere on your confirmation - you can always ring them up to ask for the PNR. They would also be able to quote you the baggage fees (though be warned - third parties sometimes inflate the costs so they make a little extra for themselves).

In the case that you are flying on KLM but your ticket was somehow issued on Virgin stock (the ticket number starts with "932" and the flights are likely coded as VSXXX) then it's possible that you would only be able to pay for extra luggage at the airport.

If, however, it turns out that you are flying on Virgin Atlantic, then it's not KLM that you should be turning to to add luggage.