To access the CodeHS Problem Bank, go to https://codehs.com/library/problem_bank
Once you have selected the question you want, click on the question.
In the top right, you will see "Copy Activity To..."
Select "Custom Course" from the drag down menu.
A list of your custom courses will pop up. Select the course you would like to add the problem to.
Once selected, you will be taken to the Custom Course's page, with the activity added.