advowson: This is not supported in game. Some cheat tools can import and export characters, including across the singleplayer/multiplayer divide. However, you may not want to do this. Single player characters always start with all levels populated with monsters, and without the ability to jump directly into deeper levels, so you will need to fight through the early levels, but your imported character is probably too advanced for those levels to yield competitive items or decent experience.
However, if you just want to be able to play the character when your friends are not around, that is easy. Start a multiplayer game, choose that character, and go. The game does not require anyone else to join before you enter the dungeon.
Thank you for the information. I will launch the multiplayer game without my friends being present. The only other reason I wanted to migrate characters was to obtain certain unique items that are not available in multiplayer. Are there cheat tools that can import those items (for example, The Butcher's Cleaver)