How to stop work item approval when payroll is locked

Hi, A

Am new is SAP workflow, got my certification last month, we have implemented a custom workflow for overtime approval which is working well, However, during the payroll period, approval happen but the overtime record in PA2002 is not unlocked, this leads to many employees not being paid and we are forced to unlock them manually, kindly assist on how i cant restrict workitem approval when payroll area is locked. i have found the interface i need to implement , IF_SWF_IFS_WORKITEM_EXIT, but stuck on how to implement it. kindly help me out.

Thanks

Kelvin Munyasia