Retain Leading Zeros in Excel

To keep the leading zeros when taking data into Excel, do three things:

1) pad with zeros,
2) concatenate with an equal sign
3) set Entmap Off for that particular column

SET MARKUP HTML ON ENTMAP ON SPOOL ON PREFORMAT OFF
SET TERM OFF

COLUMN Empno ENTMAP OFF

SPOOL Employee.xls

Select      '="'||LPad(EmpNo,8,0)||'"'
          As
    EmpNo,
    --------------------------------
    EName
From Emp;

SPOOL OFF
SET MARKUP HTML OFF ENTMAP OFF SPOOL OFF PREFORMAT OFF
SET TERM ON

See Oracle documentation for further detail:
Generating HTML Reports from SQL*Plus