By "validation" I'm going to make an assumption that you are referring to LiveCycle Designer form validation.
As far as making use of an external js file for use in validation within Designer, you can reference "trusted" functions. These functions can be stored in files within the app.path directory and can be referenced from within your form. However, this then means that any user who is to use the form you've created will have to have a copy of the trusted code file on their local system or the form will error when it tries to call the code.
Cardinal Solutions Group