Hello,
Thanks for reaching out. Enhanced conversions can only be used on previously uploaded conversions (it must be an adjustment).
By your question, "Cannot we directly call enhance-conversions API with unique OrderId to upload Customer PII data to conversion action.", are you asking if you can bypass the initial upload? If so, this is not possible. This is discussed in the 'How it Works' section on
this page.
Regards,
Matt
Google Ads API Team

ref:_00D1U1174p._5004Q2WwIg7:ref