I don't know GB Payroll, but could you please clarify the following:
I understand that you want to add WT ZZZZ to Cumulation ID ZTXP, but has that WT existed in the RT for a long time or is it a new WT?
When you say that you want "One of the wage type should only show value on pay slip from a certain date", and since RT is created by Payroll Period, do you mean that you want WT ZZZZ to be part of Cumulation ID ZTXP as of Payroll Period YYYY-PP?