Ok, you'll have to use our manual journal entries to enter this. If you' are not familiar with manual journal entries you would be advised to get your accountant/book keeper to enter it for you. You can find Manual Journal Entries in the top nav under Accountant & Reporting Tools or clicking the link. Hope that helps.
It's not a very common task so we didn't build it into our workflows. The journal entries are used to 'hack' the system, it needs to be done right so we can include it in your VAT return. If you don't have an accountant let me know and I can introduce you to one.