You may wish to check whether you've lost the zero during the conversion to
csv, or whether your problem is only if you try to read the data back from
the csv into Excel.
If the leading zero was visible in the original xls file from which you
produced the csv, then have a look at the csv with something like Notepad
and see if the zero is still there. If it is, and you want to read it back
into Excel without losing the formatting, rename the csv to txt and tell the
text import wizard to treat the columns as text, or otherwise impose the
text import wizard by importing your csv with Data/ Inport External Data,
rather than with File/ Open.