State tax filing

So I ran the numbers and it turns out that taking 2x of the Child Care Credit from the Federal & PA credits ($1200 x 2 = $2400) is the better deal for us -- especially since PA taxes FSA contributions. But now I am wondering whether I can pay taxes on the $5000 contributed to our FSA after the fact so I can be eligible for the full Child Care Credit. Perhaps by adding $5000 to Boxes 1, 3, and 5 of my W-2? Is this allowed? Is there a better method?