Yes a highelf can become a necromancer. However, do *not* attain a level higher then 17 before you betray Qeynos. I would suggest attaining level 15 or so then starting on the quest to change cities. I have never done the quest, but I can't see it giving more then 2 levels of exp before your new home is FP. Criimson Nightsky Lavastorm
I've heard that you can enter the other city without becoming a citizen. If that is true then maybe you can talk with the NPC who gives the subclass quests.
you may be able to enter certain areas, but if you approach Freeport without being a citizen guards will attack and kill you on sight so be careful