You know that very fine line between reality and fantasy that we're not supposed to cross? This adventure crosses it. Before playing, find out exactly who will be playing. Make up a character sheet for each of the players that represents the player him/herself. Don't roll their ability scores; just assign what you think is fair. Remember that they will have lower scores than regular characters--especially Strength. Don't worry about gear; whatever the players have at the table is their gear, including dice--I'll explain the importance of dice later. Proficiencies are important, however. Whatever the player knows, his/her character will know (however, languages like French and Spanish, or knowing how to change a car's tire won't be too useful, unless trying to impress NPC's with stories of the future). You'll have to use discretion with this.
Start the adventure by sending the party on some quest, like recovering the Amethyst Orb of Karnin Daganh or something. Be creative. Have them recover it and get it to where it's supposed to go. Just when they think that they've found out what the Orb does and the adventure's over, have an NPC wizard get into a fight with them and cast a spell on the Orb (or whatever creation you have come up with). Something will happen to it--it glows, vibrates, sings, whatever. Then there will be a bright flash of light, and presto! Now is when you bring in the new 'player' characters. Each one will be standing with their respective 'regular' PC's. Tell the group that there are some people with them now. The people are dressed funny, and look weak--they have no weapons or armor (unless one of the players actually was wearing armor. I doubt it, though. . . ). Describe them as looking like the people gathered around your table. If someone asks if they do anything, reply by saying, "I don't know. Do they?"
There will no doubt be tensions between the two groups of PCs. They will probably form two groups, and one will tag along behind the other.
Here's where the dice come into the game. Suppose the normal PCs get into a fight. The new ones will probably hide behind a tree or something. The normal PCs will miss every attack they make, unless the new guys get smart and realize the connection between the game rules and the game world--they must roll the dice in the game world to make the regular PCs attack! Same goes for ability checks, saving throws, etc. So, to recap: if the regular PCs (the ones that started the adventure) want to do something, the new PCs must roll dice IN THE GAME WORLD! You (the DM) can roll to determine the outcome. If the new (player-specific) PCs want to do something, the player rolls for real, in the real world, just like it always used to be.
Now the adventure really starts. The wizard who attacked the Orb in the first place has it. With it, he can take over the world. Then the Prime Material planes. Then the Ethereal, the Astral. . . you get the idea. The group must stop him. If he manages to get control of something, all the creatures living there become his personal slaves, and will sacrifice themselves and others to stop the PCs. Sounds like fun, eh? An adventure like this could span months of playing time, so be ready.