how can I use UNIX su (switch user) with SmartFTP

On a server I use, I required to login with 1 id and the use su to switch to another id (which has a password and gets prompted for when used). How can I do this with SmartFTP?

login using SmartFTP and my normal user id and password
then I need to issue the UNIX command

su - <new user id>

then a prompt "password: " appears
then I enter the password and press enter


What you need is a telnet/ssh client.