Cameron Douglas - who was sentenced to five years in prison in April after he pleaded guilty to possession and dealing methamphetamine and cocaine - entered the plea to a narcotics possession charge and has agreed to serve an additional year in jail.
He admitted Wednesday that he managed to get drugs in prison while serving his sentence.
The 32-year-old is believed to have owned up to possessing drugs from May through to last week after illegal substances were found in his cell.
Following his admission, Federal Judge Richard Berman questioned the security at two federal prison facilities.
At Cameron's trial in April - which was attended by Michael and his ex-wife Diandra Douglas - he was given half of the 10-year sentenced he faced and credited for eight months already served.
The DJ and actor told the court of a wish to turn his life around and become an "entertainer".
He explained: "I apologise to my family and my loved ones for putting them through this nightmare of my making.
"I ask for an opportunity to be a role model to my younger brothers and sisters."
Cameron, who has battled drug addiction since his early teens, has the option of taking drug education classes and will receive drug treatment towards the end of his sentencing.
Douglas was arrested in 2009 for dealing crystal meth out of the Gansevoort Hotel in the Meatpacking District.
He was arrested for a second time when his girlfriend tried to smuggle heroin in an electric toothbrush to him inside jail.