I want to move my single player hero over to multiplayer, but I can't seem to figure it out. I've tried renaming the save file from single_0.sv to multi_0.sv but when I load the game it doesn't recognize them. Any ideas?

As far as i know this was never possible with any the diablo games. The file probably has some code embbed into into it that won't let you play it online regardless if you rename it.

Could be wrong, don't know, wish you luck though.

I appreciate the information. It's good to know it can't be done so I can just stop fussing over it. :) Thank you.

it CAN be done but requires programming knowledge

Like qndel said, it's possible but require a bit of developer work. With using DeviutionX converting the hero it self is pretty trivial, in EncodeHero() you just add:

gbIsMultiplayer = true;

All you items will morph, but you can generate new similar ones using a debug build.