Can I Make a Reservation for Earlier than I Plan to Camp?

NO. Reserved sites must be occupied by the reservation holder, starting on the arrival date listed on the reservation, and the site must remain occupied by the reservation holder for the entire duration of the reservation.
You cannot setup a tent or park a trailer and then come back at a later date.

Also See:
Campsite Must Be Occupied Overnight
No Show Policy – Effective 10pm on Check-in Date