We could probably free up quite a few spaces by releasing people convicted of trafficking or posession with intent to distribute cannabis products (marijuana hashish etc.) when there are no aggravating factors such as sales to certain minors or involvement of minors in the business; or use of actual violence in furtherance of the enterprise. Also persons in prison for mere possession of anything ought to be released as unless they want to stop they wil be right back on the dope the moment they are released and keeping them locked up is just postponing the inevitable, and using tax dollars at that.
So-called "drug treatment" in prison is nothing more than a Kabuki that an addicrt will dance to gain his or her freedom. I once was in love with a junkie who was on Methadone. I persuaded this junkie to get totally clean and free. This junkie then stayed clean for a week, fell off the wagon, tried - as junkies will - to get me to partake of that hell-dressed-as-paradise and when I refused, dumped me.
This junkie died of an overdose.
If they don't want to get clean and stay clean they will not. Give their bed and prison meals to people who need punishment.