I think you need to clarify a few things before we can answer your post appropriately...are you:
1) already accepted to an MD program and thinking of going MD/PhD? OR...
2) (as Q suggested) asking about future reference (i.e. if you got into an MD program and wanted to try to get into their MD/PhD program at a later date)?
If the case is 1 and you are already accepted to an MD program and want to apply to that school's MD/PhD program, there is usually a procedure where you can apply internally and "transfer" into the program. However, if the case is 2 and you haven't applied yet, this is not the recommended route. The internal MD/PhD application route is often much more difficult than applying straight to MD/PhD programs b/c most schools have few if any spots for internal MD/PhD applicants; most of their spots are reserved for external MD/PhD applicants.
These procedures vary from one school to the next, so again it would help to know which type of situation you're in.