Several questions: pepakura/foam/blender oh my

[TankReady]

Good evening!
I have a few questions since I'm kind of a noob, so far I only used the files I found on the net, I opened them with pep and printed them to do paper models.

Now I wanna take it a step further:

 - How do I modify a pdo file? Example, there's a berserked helmet I'd like to do, but I don't want the horns, and I wish to modify the file to remove them and put a "plain" surface where they are. Is that possible? My guess is no - so

 - Which software allows me to do what I mentioned above?

 - How do I go about using a "paper" file for a foam project? So far I never built anything out of foam but I noticed usually foam files have less details and are usually "one sided" and then mirrorer for the other side. How do I know how I can alter a paper file to make it into a foam-able one? (example being the same berserker helmet I mentioned early, how do I join the faces into something that can be done properly in foam?)

Thanks you all!

[penguinopph]

Do you have the free Pepakura Viewer software, or the paid Pepakura Designer? With the latter, you can modify models.