Switching user in AIX 5

I need to do a switch user in an automated mode and do a ftp using that switched id.

Scenario:
initial login xx.
switch to user-yy without manually entering the password.
ftp some files from user yy to another user zz - automated mode.

Can any unix experts can help me for my above query?

Check for "sudo" since you are not root and promped for the pw when trying to "su", as I wrote in your other thread.