I5 2500k in my ASRock H61M/U3S3 LGA 1155?

Can you put the Intel® Core™ i5-2500K on the ASRock H61M/U3S3? Yes you can. But another question you should be asking is can I overclock the Intel Core i5-2500K on an H61 chipset based board? No the only chipsets that will allow you to overclock the Intel Core i5-2500K are the P67, Z68 and the Z77 chipset based boards.
 
on h61 mobos, the most you can do is very basic overclocking like changing to the turbo boost speed and such. If you were interested in CPU overclocking, then you would have purchased a p67/z68 mobo back then or current gen z75/z77 mobos. Its essentially one of the things you had a choice back when you made the budget decision, and overclocking was one of the decisions you cut in order to keep the price down low.