For what I understand, you can add luggage even up until boarding. However, the later in the process you add this, the more expensive it gets. I think a large hand bag starts at 14 euro or so when added at the moment of booking, but can be 35 euro when booked at the gate or at the check in.

More information on luggage here:
http://wizzair.com/en-GB/useful_information/baggage

And the price list can be found here:
http://wizzair.com/en-GB/useful_info...n/service_fees

And finally, you can calculate how much your bag would cost when bought at booking:
http://wizzair.com/en-GB/WizzBaggageFeeCalculator