The chief judge of a circuit can assign a circuit judge to hear a district-court case, if they deem it to be in the public interest. (I don't know anything about how often this is used, or what the reasons typically are, though.)
In the federal court system, it's not unusual for circuit (appellate) judges to volunteer to be assigned to preside at trials in the district courts. This is especially true if the appellate judge didn't previously serve as a trial judge. That was the case with Richard Posner, who was a highly-distinguished law professor before being appointed to the appellate bench.
The reverse is also true: It's not uncommon for district (trial) judges to volunteer for temporary duty as a circuit (appellate) judge. They do this to get a first-hand perspective of how the appeals court will evaluate their work.
