Hi,
If you are using BAPI to transfer PO, you have to make sure that the BAPI itself contains a field which could contain RFx number. For SC number, I think we have such field in BAPI, and you may just do the mapping in the BADIs. However for RFx number, I am not sure, and you may check the BAPI data from your side. If not, you have to create some Z-field to store this value and update it in ECC PO.
BR,
Ivy