This depends on what about them you want to customize, if you want to change the item's name, damage for example just extract items.txt with mm8leveleditor and all that info is in there, (as with other adivce I always recommend you first view the file with the editor because everything is in a nice neat graph form, once you view it as a text file things are still easy to find once you get the hang of it, but the rows and colums are not going to be perfectly lined up anymore) simply change what you want, then import it.If you are trying to change an items appearance entirely, this is a little more tricky, you have to find the items graphic in icons.lod, extract it, make your modifications with some sort of graphics program (fireworks, corel, etc) then import the file with the editor, however you will most likely have to modify the x,y coordinates found in items.txt in order for your character to be holding it properly, this will just be a trial and error process, unless you make the item roughly the same size shape as before.
As for switching dungeons around, yes you can do this and I can tell you how, but first I need to know how experienced you are with the editor as this isn't necessarily hard but you do have to know how the editor works in order to do this. Also you can do a search of my name (Otter) on this board and you will see my other posts where I explain how to do alot of the things you ask, or as in the case of when I was just starting out the posts where I was asking other's how to do things, Versenwald taught me how to switch dungeons and I thought his explaination was very good. Good Luck
