Is there a mod for gta sa singleplayer where cj can arrest people?

Yes, there are mods for Grand Theft Auto: San Andreas that allow CJ to arrest people in single-player mode. One popular mod is the "Police Mod," which enhances the police experience and includes features like arresting pedestrians. You can find these mods on various modding websites, but be sure to follow installation instructions and check compatibility with your game version.