Is it possible to enter a voucher with tax area and tax expl code as blank but with tax amounts?

For an Australia based company, we need to be able to capture taxes for some of the APAC countries like Fiji, Cambodia etc. under a single tax account. We were looking at the possibility of not having to enter a tax area and tax expl code when processing an invoice but only specifying the taxable and tax amounts and the system would post the tax amounts to the account set up in the PT AAI. Is this possible? I tried but I get an error that the tax expl code cannot be blank.

If not, can we use a generic tax area say 'GSTOS' and use it (along with tax expl code 'V') for processing all overseas invoices and being able to enter different tax amounts (based on invoices received)? I understand that one way to do it is to set up the tax area with a most common tax percentage say 10% and then have the tax tolerance set up high enough to accommodate tax amount variations amongst different countries but we are not looking at setting up a high tax tolerance. If I leave the tax % in this generic tax