how do I add new stuff (eg new monsters, weapons, props, textures, etc.) in my wad on doombuilder2?
sorry for my bad english, I'm from Brazil and I'm using google translator.
You can't do that with Doom Builder, Doom Builder is a map editor. Use XWE or SlumpEd or Slade or something. Check this page and scroll down to the WAD editors.
Add the monster/sprite/texture wads to Resources when loading your map, that will allow you to access them while mapping - though at some point you'll need a wad editor to import monsters, textures & etc. into your wad.