Validating Imported Transactions using the Pre-Processing Extension

I have created a custom Transaction Source and custom PL/SQL package procedure which I specify as the "pre-processing extension" for this source in the Transaction Source setup form.  When I import transactions of this source, my custom PL/SQL package validates these transactions (it performs validations which the standard import program would not perform).  If any transaction fails this "pre-processing" validation, I update all of the transactions in the batch being imported with the status "PO" (in the column TRANSACTION_STATUS_CODE, per the instructions in the Projects User Guide).  I then want the import program to leave these transactions in the PA_TRANSACTION_INTERFACE_ALL table without importing