There are two parts to this problem -- How do you find blank cells in an Excel WorkSheet and fill them with predetermined values, and once you know how to do that, how do you implement this using LabVIEW instead of doing it directly in Excel.
Before worrying about the second part (which, while not so difficult, is not something for a LabVIEW "beginner"), think about how to handle the first part, say, in Excel, itself. Write down an algorithm, taking into account (a) how to determine which cells to test, (b) what test to make (or even if you want to make a test), and (c) how to determine what to put into the cell.
Come back when you've done at least this much (and spell out the algorithm).
BS