Iím assuming your house sits on a slab, and has no basement/crawlspace. The slab should be thick enough, so adding more probably would not be a problem. As far as bonding, the existing concrete probably would need to be etched with an acid used for this purpose. I canít remember its name. It will remove all the dirt and stuff from the surface, but has really strong fumes, not good for inside a house. Also, the new concrete would have to be poured all at once, with appropriate rebar/screening to keep it from cracking. This would not be a good DYI project, if that is what you intended. With all the mess of the acid and having concrete brought into your house, consider this alternative: build a new floor out of wood. Depending on your area this would be much cheaper, and relatively easy to do yourself.
Simply build a standard frame of 2x4ís or 2x6ís 16"oc. Fasten the wood to the concrete with a concrete nail gun, available at most tool rental stores. Since the lumber is smaller than the normally required 2x10 or 2x12 floor joist, it will need to be supported almost all the way down its length. Then cover with ĺ" T&G plywood or equivalent. Some combination of dimensional lumber, on side or edge, and the plywood, should get you very close to level with the rest of the house.
You might need to rip the dimensional stuff lengthwise , but you also have the option of ľ" sub-flooring to get it exact. Adjust accordingly if you plan to do hardwood in the room, this may allow for not needing ĺ" plywood. If all the rooms around this one have or will have thresholds in the doorways, really exact height may not matter.
As always, consult your municipality for advice and code adherence, and get the required permits. They can be your best resource to help you do it right.