Hi Jutta,
One option, if it fits your need, could be to customise the quickfile invoices. Have a look below:
If not, then you could may use the quickfile invoices just as dummy-invoices, only for quickfile. But you should add a note or so to which original invoice does it belong. You could also upload your original invoice to quickfile and add it as a document to your dummy-quickfile-invoice. But I am not sure how happy hmrc is with this solution when you have got to different invoice numbers. For that reason you may should change the quickfile invoice number to your current invoice number so that both documents have the same number.