Hello,
i advise you to check /SAPSRM/BD_WF_SC_RR_PGRP BAdI implementation in order to understand how is managed:
- area type (instance of /SAPSRM/CL_WF_AREA_XXX to determine the responsible approvers: it cannot be a standard one as none deal with accounting category...),
- leading object (in your case, it cannot be the purchasing group...)
- area guid (not unique as SC items have to be validated by different approvers)
It does not mean you have to copy/paste standard ABAP code (which is used for a precise business need which is not yours...).
Regards.
Laurent.