Word of caution: going above the max stack size for an item will ban you from multiplayer servers on that character. I suggest using a separate character for SP/MP.
How so, exactly? That's not much help.
[Edited by glyth, 5/19/2011 3:57:57 AM]
If you set the editor to single player mode, there are no stack restrictions. However, as I said, you will get kicked/banned in multiplayer if you have ANY invalid stack amounts in your inventory.