There are various ways to enter integers starting with a zero:
If you want to apply a numeric format to a column of numbers in text format (for example, text "000123" becomes number "123"), do the following:
Select the column in which the digits are found in text format. Set the cell format in that column as "Number".
Choose Edit - Find & Replace
In the Find box, enter ^[0-9]
In the Replace box, enter &
Mark the Regular expressions check box
Mark the Current selection only check box
Click the Replace All button