I think it is possible in 2 ways ( you can choose the one which suits more your business requirement:
1) Add a message using SPRO configuration for adding a new custom field at header/item level.
SPRO -> SAP Supplier Relationship Management -> SRM Server -> Cross-Application Basic Settings -> Extensions and Field Control (Personalization) -> Configure Customer Fields ->Define customer display fields at header/item level
2) At coding level, you can add a Warning message in BBP_DOC_CHECK_BADI or BBP_ITEM_CHECK_BADI.
Warm Regards,
Shyam Agrawal