These steps: 1. Select the area containing those cells 2. Press the F5 function key (Goto) 3. Click Special 4. Choose Blanks and click OK 5. Start typing your formula as you usually would 6. Instead of pressing the Enter key, press control + Enter to insert this formula (using relative references!) into all selected cells.
In order to select those cells you could use find, if they all contain the same value you can click Find all. Then you can drag over the list of found items, close the dialog and enter the formula as I described already.