Actually the shuttle did have quite substantial amount of Dv in the OMS system, however it would have consumed all it's mono propellant just to get most of the TLI, not all of it. In theory, it would have been possible to add mono-prop plant tanks in the cargo bay. It is nasty hypogolics, but it's easier to handle then LH/LOX. In the bay you would also be able to carry the LM. However, you would be required to carry the entire airframe there and back. Ironically, the Russian Buran is better suited to this crazy thing, as it didn't have real engines and plumbing as the SSMEs are. All of the power needed for the launch came from its launcher (energia). Putting the Space shuttle crew compartment into capsule form would have made more sense for exploration. But that exactly the Orion CEV. same space per person as the space shuttle. And no Apollo-style "surprise bags×´.