To add a leading 0, do the following:
  1. Open the Excel file.
  2. For the cell, right-click and select Format Cells.
  3. Change the cell type to Text.
  4. Rewrite the number with the leading 0.
You can also add leading 0s that may have already been removed. Here is a link to adding the leading 0 back in to the Excel file:

http://www.extendoffice.com/documents/excel/792-excel-add-leading-zeros-to-number-text.html