from the detail room select the cabinet and right click, adjust cabinet features, cabinet end (S,J,C) select the left, right or both sides radio button make adjustments if needed to the insets, select the door you want click ok...
This will put a end panel on the cabinet. But if you want the end panel to replace the left or right sides you will need to do so in the cabinet editor and align them accordingly.
I appologize fro not reading carefully. You asked about corner cabinets. They are not supported. You will have to take the cabinet into the editor and add the door there. Don't forget to associate the door to the cabinet before you save the file.
Mike Sand is correct about the settings in the door dialog.