I don't know how your dataport is designed, but you can also use this technique:
Declare a global variable recGenJournalLine that point to Table 81.
"OnBeforeImportRecord()":
CLEAR("Gen. Journal Line");
"OnAfterImportRecord()":
CLEAR(recGenJournalLine); // clear the global variable
recGenJournalLine."Journal Template Name" := "Journal Template Name";
recGenJournalLine."Journal Batch Name" := "Journal Batch Name";
recGenJournalLine.SetUpNewLine(); // Function used in the table 81 to setup anew record
recGenJournalLine."Line No." := "Line No."; // you can also do a find('+') on the table to find the last line no. and then add 10000.
recGenJournalLine.VALIDATE("Field1","Field1");
recGenJournalLine.VALIDATE("Field2","Field2");
...
recGenJournalLine.INSERT(TRUE);