If it were not a crime to steal from criminals then everyone in prison would lose their property rights to possessions they hold in the outside world. All of their stuff would be subject to confiscation.
In the US this would be illegal. The US Supreme Court has a special topic area devoted to resolving issues concerning prisons and jails. No doubt folks would not be surprised to learn that prisoners in the US are sometimes murdered by guards or other prisoners. So issues like these, and serious injury, are sometimes referred to the Supreme Court. Over the last century or so, a number of cases have resulted in decisions that are now part of settled law. Among these is that prisoner's retain "a constellation of rights" while incarcerated.
In short, no one can deprive a prisoner of life, liberty or property without due process of law. The sentencing court--assuming it lawfully followed due process--could not reasonably dispossess the convict of property except under certain circumstances. For example, a vehicle used by a drug dealer may be subject to impound and auction. But in general, prisoners retain the right to possess property while incarcerated. And their rights are normally fully restored upon completing their sentence.
Under the rationale of the Supreme Court it would be a crime and/or civil tort to take a person's property simply on account of a criminal conviction.
