I am no expert on this, but from what posts I have read, no. You must order your AP in advance. You must use your number from this, to order the DDE in advance. I'm sure someone else can and will give you more detail, but just wanted to give you a quick answer.
Once you have your AP, you can call and order your DDE...but remember the DDE office where you have to call to order your DDE is only open Mon - Fri, closed holidays and weekends...so you may not be able to get the DDE in a timely manner, if you arrive on a weekend or holiday.
TLSnell is mostly right-- although one could buy a DVC AP on-site, the only way to get the DDE card is in advance. You will need an AP number or at least a voucher number.
Since you arrive on a Tuesday, you can call DDE as soon as you purchase your AP. You can then pick up a temporary card at Epcot after 5:30 on Tuesday or Wednesday (depending on arrival time on Tues.).
TLSnell is mostly right-- although one could buy a DVC AP on-site, the only way to get the DDE card is in advance. You will need an AP number or at least a voucher number.