Automate ineractive applications

Hi Friends,
I have a ksh script which is created by a user, 'ustst'. I need to su to another user, 'ghprod' and execute a script created by 'ghprod'.

su - ghprod
The above cmd asks for ghprod's password. I need to automate such that it takes the password form a hidden file without prompting the user.

I am aware of Expect tool but it is not installed in my server. Can anybody suggest a way to automate this without using expect.

Thanks in advance,
Sudha.

add entry in $HOME/.ssh/known_hosts file