Thanks. By the former, I assume you mean creating them as a PC using DDB's Character Builder. If I do that, the character has to belong to the same campaign as the real PCs in order to appear as a token. And that breaks the Encounter Builder a bit, because it assumes that you are creating encounters for all of the active characters in that campaign. Unless of course I'm doing something wrong or have missed something for one of those tools...You can just create an NPC as a character and import any art that you want; that should allow you to call up a token for them. Alternatively, you could use an existing creature as a template for them and edit using the monster creator feature; this includes editing the art if desired.
I'm currently doing exactly what you describe as the "alternatively" option. It works okay, but is a slightly long-winded way of adding a token.