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 "Playlist" from the drag down menu.
A list of your playlists will pop up. Select the playlist you would like to add the problem to.
Once selected, you will be taken to the Playlist's page, with the problem added.