Can you get additional money for agreeing to donate a kidney to someone?

Against the law. Payment for an organ, or exchange of "valuable consideration" is prohibited by the National Organ Transplant Act (NOTA)in the US and be equivalent laws in all other countries except Iran where it is legal. But your expenses (travel, parking, childcare) can be legally covered. In the US, the govt and the asts are actively studying whether payment for such costs removes a reason not to donate .
No. It is a federal crime to receive valuable consideration for donating an organ (the national organ and transplantation act 1984). http://history.nih.gov/research/downloads/pl98-507.pdf.