add 2nd baggage

hello all, I confirm my ticket on 6th Sept-2011 from Skavsta to Milano. I have booked (paid the expense) one hand bag of 10 KG and one checked luggage of 20 KG. Now I want to add another luggage of 15 KG. hope I can do that ! When I can add this luggage by www.ryanair.com ? I mean how many hours before the flight ? and If I want to change date of my journey can I do that? if yes, how ?
tack

'Manage My Booking'

can you please tell me is that ok 10KG in hand bag ? or It has to 8 KG?

It's 10 kg max for the cabin baggage :

Strictly one item of cabin baggage per passenger (excluding infants) weighing up to 10kg with maximum dimensions of 55cm x 40cm x 20cm is permitted. (handbag, briefcase, laptop, shop purchases, camera etc.) must be carried in your 1 permitted piece of cabin baggage.

As long as you don't "Online Check In" you can always change your booking.