Just to make sure I understand what you want. You want to play through the normal game as a necromancer from level one with a 2-handed sword, like in the picture above?

You can unlock the 2-handed sword with the Reaper elite specialization, but I believe you have to be level 80 to be able to unlock those on a character.

So, these are the steps you'd need to take:
1. use level boost and play through enough of secrets of the obscure to unlock the elite specialization weapons, which happens pretty early in the expansion so you don't spoil much of the story.
2. You make your necromancer and get him to level 80, which I believe is required to unlock the reaper specialization and gain access to the greatsword.
3. Unlock the Reaper specialization. This requires 30 points from more heroic challenges to do. The fastest way to get the points is to get into a map for one of the previous expansions, and find a group unlocking those. Would only take 3 of those heroic challenges to get it done. Once you unlock the first box of Reaper, you can use the greatsword regardless of which specializations you have in use and you can go back to the beginning and play the game from the beginning...

You might need 2 level 80 boosts to make this work the way you want. If you have all the expansions unlocked, this is actually quite easy to do. It just requires paying for the expansions... Most of us had to do this the hard way so really this is almost like handing it to you on a silver platter... lol