The following article discusses formulas to use which will prevent the 0 from dropping instantly. However, once the file is saved and closed, re-opening the file will drop the leading zeroes again.

One way to avoid this complication would be to initially have the data saved as an Excel spreadsheet, which will keep the formulas without a problem.
1) After saving the Excel file with the correctly formatted data, click File then Save as a CSV.
2) Do not open the CSV file (as that would result in the numbers reverting and dropping the leading zeroes)
3) Right click on the CSV file and edit with Notepad (or Notepad++).
4) Notice that the file still has the correct data and that values are still separated by commas.
5) Save the file as a .txt file and use that to import.