Yes you can and up to the last minute too.
I did this for my trip last week, i was going out on the 4th May and booked the vouchers on the 27th April. They ask if you want to pay for it with whatever card details you gave them to pay the initial trip too and then just add it to your invoice on top of the room cost.
Besides, because as you have booked room only they don't have any park tickets to give you-it gives them something else to stick in that little envelope with the hotel gumpf!