Yes, you can enter Friday Practice as an Endurance Team even on non-endurance weekends, per the following clipped from a reply I posted in the Member Section:

Quote Originally Posted by Alan Etheredge View Post
..... If you want to use the Endurance Friday Practice form on a non-endurance weekend, just fill that form out from the "Endurance Pre-Entry Package" and submit it with your other sprint-weekend forms. You'll need to manually write-in the event-date, I didn't anticipate this so the event-date drop-down on that form only has endurance weekends (sorry about that ).
For more info see the whole post here (login required): Endurance bike practice during non-endurance weekend?