With your keycard you can buy a skipass at every moment.
It is possible to buy a skipass online without a keycard, just selecting the option "I have not a keycard".
Once your order is done, you will get a QR Code to catch your keycard at the Espace Prarial automatic machines (in yellow in the map below).
Less than 2 days befor your arrival, if you have not a keycard you can not purchase a skipass online; but don't worry! The keycards are available at the cash desks, always at the cost of 3 €, and you can ask to the casher to load your skipass on the new card, that you can keep and use for your next online purchases.