How to convert from String Array [] to byte array []

Sorry for missunderstandings . I made a labview program which sends data through serial port . I have 8 bytes , so 8 values . I used the String object to be able to store FF as a single element, not two .Then in arduino i want to take these 8 bytes and pass them to a function which only accept byte array as argument .
In the byte array i want the equivalent hex values (FF)