yhw42 pretty much answered the question. However, there are two things I'd like to add.
First: Shadow power source
The DnD product catalog entry for Heroes of Shadow (scheduled for March 2011) lists a necromancer class to be included in that supplement. But I can't say if that class is going to include undead summons or not, sorry.
Anyway, if you want to reflavor a certain aspect of the game the DMG encourages you to go ahead and change the description/appearance of your powers or feats or magic items (as long as it doesn't give you any additional advantages beyond an altered appearance). So you can certainly make all summons from Arcane Power appear as undead or purely elemental creatures if they aren't already.
Also, if your DM agrees you can also change damage types. There's that example in the DMG of swapping all paladin powers that deal radiant damage over to necrotic damage if you want to play a paladin following an evil deity. However, in that case the DM should definitively know what he's doing because changing damage keywords can really alter balance. It can open up horrible loop holes (side effects with certain feats or items) or leave the character weaker than intended (e.g. switching too many powers over to necrotic damage in a campaign with many resist necrotic monsters).