Within CB > User Management select the checkbox for your user then use the download button top right of the table. This will give you the CSV scaffolding to start with. You'll then need to format the data you want to import into that CSV. Once done use the import button next to the download button top right of the table.
Note when opening our export CSV be sure to use the following settings in your editor.
1. Character set: UTF-8
2. Separated by Comma
3. Format quoted fields as text
4. " as string delimiter
Joomla also will require a name, email address, and username to create a user so be sure you supply all 3 during importing.