It is currently only possible to have one net to gross component and it must be a fixed value.
We have clients that we cannot migrate as they use hourly based N2G. We would not be meeting our legal obligations to provide an itemised payslip if we were to process as a fixed value unless we went through the long admin task of amending each line item with the personalised breakdown which is not something that we have time for.
We also have clients that require more than one net to gross component which is causing restrictions - I know that this idea has already been raised.