What is the pad for?
You will need to score it so the new concrete will adhere to the existing. Then frame up to pour and finish. Depends on size and purpose as to how thick the new pour layer needs to be.
Score so the new ready mix will adhere. Frame up and pour and finish. Depending on size and purpose as to how thick the additional pour needs to be.
How off level is it? New concrete will not hold to old even with scoring. Also the added weight of new will cause it to sink more.
Sandjacking. Or if you're up to it, there's YouTube videos of how to do it yourself with a couple of jacks and sheets of dense foam insulation.
William is right, concrete will not adhere to concrete. Period
For a temp solution, sure. It will look better but You sure the hell won't be able to use it for anything load bearing ie: parking a car
Is the pad not level because of the way it was poured or because it is sinking? You can add another layer to level it. You would need to use an acid etch and then a bonding agent otherwise it will delaminate, but as William mentioned, it does not solve your sinking problem if that's why it is not level.