>>29412The Civil Protection are pretty explicitly stated in-game to be unaugmented humans who volunteered to be part of Combine's occupation police force in exchange for better access to food, water, medicine, protection of their family members and VR pornography, or whatever "non-mechanical reproduction simulation" could mean. During their service, they are gradually brainwashed and sterilized, until the point the Combine command deems them loyal enough and direct the metrocop to Nova Prospekt for "promotion", where the subject's memories and basic instincts are overwritten with Combine's commands, their bodies are implanted with basic synth technology and then are transferred to the Overwatch units.
As for Barney, my theory is that he managed to evade the brainwashing and being turned into a cyborg because he likely was a new member of the Metropolice and did not take any mind-altering drug laced rations the Combine has paid him with for his service.