This post was in regards to Diablo 1. I'm not sure about the current version of Diablo 1, but in the past versions the multiplayer characters were stored in the Windows installation directory as hidden files (dlinfo_##.drv). These files were in MPQ format, so importing them into single player by just doing some simple copy/pastes was impossible. If this is still the case with the current version of Diablo 1, the only way to convert your character easily is through the use of some Diablo trainer/backup program, which is of course a third party application and I wouldn't be surprised if many have trojans/backdoors in them as well. About the only advantage D1 single player offers over D1 multiplayer is a few added quests, if this isn't a big deal to you, then I would suggest playing LAN play (I'm guessing you just don't want to play on BattleNet).
-- Magickian, because I'm too lazy to login