From the OSR - TxnLineID
TxnLineID Identification number of the transaction line. (TxnLineID is supported as of v2.0 of the SDK. With qbXML v1.0 and v1.1, TxnLineID is always returned as zero.)
If you need to add a new transaction line in a transaction Mod request, you can do so by setting the TxnLineID to -1.
So enter -1 to add a line.
<SalesOrderLineMod>
<TxnLineID>-1</TxnLineID>
... the rest of your new line item details here ...
</SalesOrderLineMod>