I have done this exact thing before, but before I go into an explanation, I have some questions:
Will (can) the new porch floor be higher than the top of the existing concrete? If so, by how much can you raise it? Are you planning to support a roof or other structure over the entire new porch? Do you live in a climate that receives snowfall? How old is the existing concrete? Does the existing concrete have a footing, or can you tell how deep the concrete goes?