Hi Victor,
actually, you are allowed to enter interest rates, though I might agree that this is not the most convenient way to do it.
first of all, you need to enter any (dummy) amount, e.g. EUR 1 (let's say your deal currency is EUR), then you go into detailed view and override it by specifying all the parameters, period from/ to, calculation base (nominal); interest rate, etc. - and finally select the icons 'Pro rata temporis' and 'Copy' to update/ transfer that information into the transaction you are editing.
Kind regards,
Renatas