Just download newest MCEdit (it must be a version that works with Anvil, otherwise it will say your map is empty) double-click on spawner and write EntityID (in this case Skeleton) you can find IDs on Minecraft wiki.
Well, I don't like posting in old threads, but since this one has been resucited, I can say that yes, you can use NEI to get any mob spawner in the game, with any mob, vanilla or from a mod. I guess this could be useful for some map maker who comes to this thread using google, so... here is the answer.
Rollback Post to RevisionRollBack
Juega las cartas que te da el momento. Mañana es solo un adverbio de tiempo.
If I remember correctly, it is stored separately as an NBT tag associated with the block. you would need either a worldedit filter or NBTedit/other external program to be able to set the spawners. After all, you can make mobs spawn with armor, different weapons, and enchantments on that armor. that won't all fit into a block ID, so the guys at mojang use NBT files instead