Yes, if you are certain you didn't pay anything for these shares, then you can enter "0" as the Cost Basis.
Before doing this, check with your employer's payroll department and make sure that the company did not include any "cost" for these shares in your taxable income (Box 1 of your Form W-2). You may not have seen any change in your paychecks, but your income taxes would have gone up). If they did "add" the cost of these shares to your taxable income, then you really did pay for the stock - you cost basis would be the amount they added.
