You can either apply a custom format to the cell of "000000", in which
case the value in the cell will still be a number, or you can set the
cell to a Text format and re-enter the data (or enter it with a
leading apostrophe like '000702) in which case it will be a text
value.
If you have many cell values that needs to be reformated you can
insert a new column and write the formula =right([cellreference]
+1000000;6). If the value in the cellrefrence is 702 this formula will
return "000702" as it returns the 6 characters from right of the value
"1000702".