Unfortunately, it is not possible to add a new class after the trademark application has been filed or the trademark has been registered. The reason is that adding a class changes the scope of the protection you would be applying for, which means your application would have to be reevaluated.
In case your trademark application faces opposition, it is possible to drop one or more classes that pose an issue to continue with the trademark proceedings. This limits the scope of your protection, meaning your mark and the opposing trademark might not be seen as overlapping and potentially confusing to the consumer.
If you wish to add more classes to your trademark, you can simply file a new application with the same brand name or logo. This way, you will have two or more trademarks covering different goods and services. This is a standard procedure utilised by companies and individuals that expand their portfolio of goods/services and wish to fully protect their business.