I'm not sure if the UserProfileService will return the user's department. This will vary from server to server, depending on configuration/settings.
Regardless, once you have a data connection to receive user data, you can take a look at the data being returned by the secondary data source and use it as needed in your form. Since the data connection brings the data into the secondary data source, all you need are rules to set the values in your main data source.
Good luck!