I'm a bit confused - are you wanting to make a zombie/skeleton of a bigger creature (like a giant, or something?), or create a zombie of an existing character? Or else, just increase the HD of a plain vanilla zombie monster?
If it's the first two, you can just add the zombie template to the creature/character you are after.
If it's the last case, you could add the Advanced template. It doesn't actually increase HD, but it increases most other stats and makes a generally nastier critter.
It's probably easier to do it the first way, however - create the mortal character first, then zombify them using the template and you'll essentially get what you're after. I think!
