The ORIGINAL Sniper Puss
Yes you can. WBB rolls are normally performed at the start of the Necron Turn, before any movement or whatever is done. If any rolls have been failed, you can indeed then teleport the affected unit and attempt re-rolls of the unsuccessful original rolls, PROVIDED THAT there are no Warriors waiting to use their special rule to come in from Reserve, which takes preference for the use of the Portal. The unit attempting the re-rolls may not move before they are teleported, but there does not seem to be any ruling in either the Codex or the FAQ to prevent the Monolith moving towards the unit in order to get the unit into teleport range, should this be necessary. If the Monolith *does* move, then there is a movement penalty for the teleporting unit.