How are Contractors made in Darker than Black? Do you have to be born with the powers? Or do you have to make contact with a spirit and make a contract with it?
According to the wiki, the process to become a contractor was never revealed, only that there is a link between contractors and the appearance of the Gate.
Shion Pavlichenko was born as contractor, and Suou became one after she absorbed Hei's powers, and, in the Darker than Black: Shikkoku No Hana manga, a character named Harvest has the ability to make people Contractors, but all other Contractors, such as Tanya, have become what they are via unknown means. In Kuro no Keiyakusha Gaiden it is revealed that Contractors are defects that emerge when humans are unable to become Dolls.
So it looks like there's any number of ways to become a contractor. Some are born with the powers, and it's also possible to obtain the power via someone like Harvest.