I think you can book your room whenever you want. Then, when you make your room block, give them your ressie number, and they can transfer it into your block. You just have to make sure you choose a hotel that is going to be in your block.
I would only pay the one night deposit, though, when you make your reservation because your total will be discounted once it's part of the block and it's silly to pay money that you will just have to be credited back