Am I able to load additional airtime and data onto a SIM?

Yes, you can.

For the USA you can add additional airtime, data, and international credits on T-Mobile’s website in the US as well as any T-Mobile stores, and most convenience stores.

For UK/Europe the best way is to download the My Vodafone App whereby you can keep an eye on your balances and top up as required.

For South Africa, you can visit any Vodacom Shop or convenience store or you can register on My Vodacom where you can check balances as well as top up as needed.