Add leading zeros to existing data in a cell

We have a report in a text format that requires all cells in one column to be filled with leading zeros prior to the existing data. Rather than type in all the zeros, is there a way to insert leading zeros without changing existing data within the cells?

I have tried using the edit/replace replacing blanks with zeros, but this did not work
Jim Partney

If the values in the cells are numeric, Format > Cells > Number, Custom, 00000 (enough zeros to pad to the length you want)