I just wanted to know whether or not you can create items with the editor and use them in the single player game and if so how.

Yes you can create items with the editor. Make an item, make a mini module with your item in it.

Save your character. Load that character into the new module. Get the item. Save your character again. Load that character back into the main campaign. I'm not sure if you can do this in the middle of a chapter, or only at the very beginning.

in multiplayer (choose to host the game, even if it's only u playing) you can pick up right where u left off, in singleplayer you need to go to the beginning of the chapter, easiest to do when you're just getting to a new chapter