Yes, you can. The pre-registration is designed to prevent queues at the gate, but it is not a hard requirement before you arrive.
You can complete the slot information directly on site. However, please note:
The pre-registration must be completed before you drive up to the gate window, as you will need to show your Slot ID there.
If you are completing the registration on site, do not do this in front of the barrier. Please pull to the side, complete the process, and then rejoin the queue. This keeps traffic moving for other drivers.