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

Yes, you can add additional airtime, data and credit:

For the USA you can add additional airtime, data, and international credits at any T-Mobile store, most convenience stores or on T-Mobile’s website.

For UK/Europe on Three Mobiles website.

And, for South Africa, you can visit any Cellucity, Vodacom Shop or almost all convenience stores.