I'm assuming the module in question is Adagio Invoices.

While Andrew's method likely works (I haven't tested it), you can also do more easily without having to create a new batch.

Add the new invoice to the end of the existing archive batch. Then highlight all of the lines you want after the new line, and copy them to the existing batch, making sure to uncheck "Increment document number" and "Revise date fields", and especially making sure to check off "Move document".

Voila!