How to Create a SES with acc Assignment Category 'F' even though Purchase item Having acc assignment category 'U'

Hi Experts...

I am creating mass service entry sheets with BAPI_ENTRYSHEET_CREATE...as per the client requirement if purchase order item acc assignment category is 'U'..then we have to Create service entry sheet with account assignment category 'F' with Multiple line items...

we have passed all the parameters as like other cases But we are getting error as 'No Account assignment exist for line item 00010'....

Could you please help me  to create SES with acc assignment category 'F' even though purchase item having acc assignment category 'U'...

Thanks in advance.