You can set up FileZilla to use sFTP by selecting Require explicit or implicit FTP over TLS. Set your port to 22 and use the instructions Bala explains above but it only works with your cpanel user name.
Purchase the domain then open a ticket and have the help desk park the domain on top of your existing one. Then your site will be accessed using either domain name.
The reason for using the server names instead of your domain name is you are using secure ports for IMAP and you don't have your own security certificate. This eliminates having to accept the server certificate each time it checks for mail.