Then here is a macro solution:
First select the block of cells you want to fill. The macro will ignore any
cells in the block that are not empty.
Then run this macro:
Sub fillEmpty()
Dim r As Range
For Each r In Selection
If IsEmpty(r) Then
r.Value = "luis"
End If
Next
End Sub
Macros are very easy to install and use:
1. ALT-F11 brings up the VBE window
2. ALT-I
ALT-M opens a fresh module
3. paste the stuff in and close the VBE window
If you save the workbook, the macro will be saved with it.
To use the macro from the normal Excel window:
1. ALT-F8
2. Select the macro
3. Touch Run
To remove the macro:
1. bring up the VBE window as above
2. clear the code out
3. close the VBE window
To learn more about macros in general, see:
http://www.mvps.org/dmcritchie/excel/getstarted.htm