Anyway to have the online dlc characters playable in the single player world?

[At@At]

Like maybe a mod or something?

[Lord Misfit]

Cheat Engine is really your only means [unless someone directly modded the game, but sadly DQH1/2 in general has so few 'mods' outside of CE]. http://fearlessrevolution.com/viewtopic.php?f=4&t=7719 has a cheat table [the top most option in the attachments is the latest version]. When you open the game, you need to turn on Cheat Engine and attach it to DQH2.exe, and then load the "DQH2" table.

Select [Enable] to expand the list, then select [Pointers], then select [Party Members] and you'll see settings for the four character slots. You will see each character slot has two settings, so it's recommended you set both of Slot 1's to the same character, and so on, but it will let you use the DLC/Dungeon-Only characters in the main game.