In order to transfer your characters from Diablo to Diablo: Hellfire, goto your save file and change the extension to .HSV. For example, if your file is called RASSILON.SV then rename it to RASSILON.HSV.
When you get the cornerstone of the world quest in the crypt, tp to town and save the game. Make a copy save file of your character with a different save #. Create a seperate folder to put the copy in. Now move your original character to your desk top or another folder and place a copy of your copy save file back into the main hellfire folder. Start up the game and load game with your copied character. Go to the cornerstone and drop the item you wish to dupe on it to activate the speech. Now save the game and quit. Move the copy file to your desktop and then move the original character file back to the main hellfire folder. Start the game up again and load your game. Go to the cornerstone and the object the copied character dropped there will be waiting for you. Pick up the item and then tp back to town. Save your game and then repeat the process again with the same copied file (the original item will still be at the cornerstone). Pick the item up and then drop it back on the cornerstone again. you can repeat the process ad infinitum. YOU MUST ALWAYS RETURN TO TOWN WITH YOUR ORIGINAL CHARACTER EACH TIME YOU DO THIS OR IT WONT WORK.
I've created entire libraries of spell books, elixirs and etc. for this very purpose. You'll never have to level again if you do this with the spectral elixir.
this one can be useful for some bosses and hard creatures, its a cool game glitch. Before you cast the stone curse spell have a tp opened up near at hand. Now cast stone curse on the target creature like 5-10 times in quick succession (wont work if you dont do this) and then duck in the tp. when you go back through the creature will still be a statue and will never turn back. easy to kill.