SD FI-CA integration

I could use a little advice on SD-FI integration.

When I create an invoice to a contract where account assignment is a WBS-element the accounting document is not updated with the referencing SD-document (contract/sales-order).

But to my great annoyance when account assignment points at a profit center, the Sales Order field is filled.

I wonder if any of the SDVFX... exits could be used for filling the accounting document ?

Any ideas ?

BR

Jens