Questions Import Scenario for AP Debit Adj & AR Credit Memo with Application by a custom key

I have a client that has high volume of AP & AR transactions from an external system.  They want to import them even when it's not finalized to show up in their aging so that creates a dilemma when we need to reverse out the estimate and replace with final transaction.  Ideally we would do the accrual in GL but they don't want to.  It's crazy but that's what they want. So I need to create an import scenario for AP Debit Adj & AR Credit Memo with application.  First issue is i can't seem to get AP Bills screen to take the application document. Has anyone been able to successfully apply a Debit adj to a bill via import scenario or even on screen in the Bills & Adjustment Screen? 

Second issue I have is...the application document has to look up by by vendor ref and customer order field respectively because the Bill and Invoice Ref Nbr isn't available in the data since it's coming from an external system.  Does anyone know how to look up the application doc ref nbr using a custom key (vendor ref/customer order nbr)?  I tried the @@InvoiceNbr but doesn't seem like it would work the field isn't available in the application object to map to.

Any help would be greatly appreciated.