We currently pay our employees semi-monthly through the current date ie: payperiod is 3/1 to 3/15 and payroll checks are dated 3/15. How do I create a lag without withholding a check from the employee?

Most employers pay a week after the pay period. Employers are allowed to have time to adjust paychecks for hours added/subtracted, sick/vacation time, etc.

You can also just continue as you are and make adjustments for the last half month in the next paycheck. The drawbacks to that are that employees get confused, and you have to make adjustments after the fact if they don't work for you anymore.