Please do not post your question to the same issue in different discussions: Purchase order, referencing outline agreements
As your issue is caused by a modification to the SAP standard via BADI or Exit it would be a rather stupid idea to do additional customizing or even a second development instead of correcting the mistake.
Also a field selection for ME22N cannot be made date depending, it is valid for any purchase order. I doubt anyway that you can get around this program check by a simple field selection customizing setting.