The death note cannot lengthen a person's life span by setting a date of death for that person after their current date of death. In this case the person would just die as they would if their name was not written. This is from "How to Use LVII".
I was wrong on one account originially. If you write a person's name in the death note and do not edit it for 6 minutes and 40 seconds (400 seconds as that is how long you can edit it), then he is immune from other death notes. This is due to "How to Use XV" which states that if the same name is in two death notes, the first filled in will take effect. This can only protect for 23 days as the person will die by then anyways. It appears that this can be circumvented by editing it every 6 minutes and 40 seconds.
You can, however, do the following. If you know Dusk is trying to kill Dawn, you can write Dusk will "be killed by Dawn after failing to even scratch her with his attacks". This means he must die of either a heart attack because the cause of death could not happen (could not reasonably get to Dawn or Dawn killing Dusk is impossible) or will be killed by Dawn with hurting her. In any case you need Dusk's name and you are extending Dawn's life by killing Dusk with the death note which humans (not Shinigami) can intentionally do.