How to install mods in the Steam version of Torchlight 2

Since Torchlight II will have completely new character classes, and none of the ones from Torchlight I will have a comingback, you will not be able to use old characters.

It is also stated in Torchlight II FAQ in Runic Games official Forum:

Will I be able to import my old characters?
No, the original characters will be in the game as NPCs, as will other NPCs of the Destroyer, Vanquisher, and Alchemist class types. Additionally, due to various changes in gamplay systems the old character files will not be able to be imported as-is, as detailed below:

Along with the story the game will use a new wardrobing system opposed to the one in Torchlight. As a result they did not reuse the Torchlight character assets.

Loot drops are per-person. I just played a LAN game for a few minutes and all the dropped equipment and gold was exclusive to me, unseeable and untouchable by other players in the session.