Pouring concrete over concrete
There are a number of things to consider here. First off, will you be raising the level of the outside porch to make it essentially a walkout from the house, in other words will there be any step left at all? From your description, it sounds as if you will be bringing up the finished porch floor to a levell where you would have concrete above the sill plate and rim joist on the existing house. If you finish enclosing the porch in a short time, that might not be a problem, but other wise you are apt to have water get trapped between the new concrete floor and the wood framing, which will result in a rotted sill and rim joist. Even after the porch is enclosed, you have enclosed an area that is no longer open for termite inspection.
Secondly, how solid is the existing slab? If it is solid and level (not necessarily without cracks), you could probably pour the new floor over top of it, but I would put down a vapor barrier rather than worry about bonding to the existing concrete. You're talking about converting what was an outdoor space into an indoor space, and unless you want a damp floor, you should consider keeping moisture out a priority.
If the existing slab has sunk or is sloped or in any way not stable, then you willl probably have more problems by adding the weight of additional concrete on top of it.
Have you considered framing out the new floor rather than using concrete? If you're talking about 6" to 10", why couldn't you frame the floor?