Sounds like you want to set a parameter/variable in the .kettle file and reference it in the File or directory
text box. Note the red dollar sign next to the box. That means this field accepts variables. Here's the wiki entry for variables:
You can also read from a config file directly (from a transform) and set it dynamically with the Set variables
step if you can only have one .kettle file. Also check out the Check if connected to repository
(from the Repository branch) step as well and see if that will suit your needs.
If none of these suite your needs, please add detail to your question to describe exactly what you're trying to do and how you're trying to do it.