When 'categorising' an expense via the action hub, we are currently unable to add invoices, yet we can add them when 'matching' via the expense hub. To add an invoice to a categorised expense, we have to navigate to the money out>expenses section and edit the expense we just categorised.
It would be much better if we could add an invoice at the time of categorisation, so that we can keep financial records for all expenses without needing to add a supplier for each.