I had the idea a while back to solve the ever so complained about backpack size. so i though a good way to alleviate this would be to create a item bank location that will hold your items. i though of a couple of ways of doing this, but have not found a way to do an of them in the modding help guides. first would to be to use the portals that go to level 0 that were added in the Realm of the Diggle gods to have them transport you no mater the level to the item bank. other ideas involve stairs to take you to a bank which are also quite lacking in all of the guide i have read up on. from what iv read transporting spells and the demonic hieroglyphs can only move you on that Z level, so their is no way to use them. people have also suggested that the item bank have a fee to using its services. This would serve as a effective money sink late game. Any input would be much appreciated.
Conquest of the Wizardlands will have a pocket dimension which essentially means unlimited space you can access at any time (on a 30 turn cooldown).
None of the ideas you have are possible. Mods can only add skills, spells, items, monsters, recipes, taunts, and rooms (that get randomly selected like any other room). You can't modify anything in the game, or change the way anything behaves, like stairs, portals, or any of that. At any rate, as Daynab said, this issue completely goes away August 1st when Conquest of the Wizardlands comes out.