This is possible to do in the FTP section of one of our control panels. If you create a sub domain stock.domain.com it will point to the folder public_html/stock unless you specify otherwise. Go to the ftp section of the control panel and create a new FTP account called stock. The username will become [email protected] with the password you choose. This ftp account will only have access to the sub domain folder.
Note: If you are using the Admin Center to create the ftp accounts you must resync ftp passwords after the account is created.