When you have employees with various 401k contribution limits, you can not alter a single 401k payroll item to reflect multiple contribution limits.
Once an employee reaches their contribution limit, this article will help you set up a new Payroll Item to meet their needs:
To create a new payroll item for each different 401k cap:
Return to the Payroll Item List.
You can do this for each varying rate and then apply to the appropriate employees to allow you to have multiple 401k caps within the same company.