No, you can't modify classes after filing your trademark application with the respective IP office. You can submit a new application, but you'd have to pay twice.
If you are using a third party, such as Trama, to prepare your application, there might be space in their process to modify your classes before the draft is finalized, but this would be before the final application is submitted with the authorities.
There's one particular case where you might have to make an edit. If your application is opposed by an owner of a similar trademark in one of your classes, one solution is to limit the number of your classes so the two trademarks won't 'overlap' in scope anymore. But adding classes won't be possible at any point.