Page 8 of the rule book:
...It does not cost any extra movement points to open or pass through a door.
Note: Doors in a wizard’s home sector are always considered to be unlocked for him. A wizard may cast a spell through an open door without passing through the door himself. No one can follow a wizard through a door unless the wizard holds the door open to allow it. Once a wizard is no longer adjacent to a door, it immediately closes and re-locks.
So, opening an unlocked door does not cost movement points and a wizard may open an unlocked door as long as they are adjacent to it.