How to derive balancing segment on tax distributions from paygroup?

 I have a need to modify subledger accounting in payables to derive the balancing segment on the tax charge account and tax liability from the paygroup rather than from the tax code setups.  I have set up the mapping but paygroup does not appear to be set up as a source.  Is the only way to do this to create a pl/sql procedure and attach this to a custom source, or is there a simpler way?

Thanks