By default, most CSV or spreadsheet programs will automatically remove leading zeros in a cell. For example, it will automatically convert 03304 to 3304. You can change the cell format to retain the leading zeros (0) in the field to allow you to upload them into your organization. 

Steps to retain initial '0' when importing a CSV:

1. Once you open the CSV file, right-click the column letter and select Format Cells.
2. Then choose Custom, and set the format to '00000000.' The number of 0's should indicate the number of characters desired in the field, including the 0's. If the field value is 012345, then the custom format should be '000000' to indicate 6 digits with the 0.
3. This will add the leading '0' to all cells from which it is missing. And when you save it as .csv, it will export the leading '0' in the output file. Note: Once saved, don’t open the CSV file to save the changes.
4. Now you can use the CSV file to import your record.