To keep the cell  value as zero but display the word NIL, try a custom format.
Select all the cells on your worksheet, then select Format >> Cells >> on the
Number tab, choose the Custom category. Enter a custom format like the
following:

#,##0_);(#,##0);"NIL"

Click OK. The above example doesn't display decimals. You could edit it before
applying it (the following version displays 2 decimal places), or change
individual cells.

#,###.00_);(#,###.00);"NIL"

Either way, if you just enter 0 in a cell, it will display as NIL but still
contain the value 0.