Yes, applications for an Extension can also be submitted from overseas. Similar to the majority of applications submitted from overseas, the UKVI Standard time is 15 working days. You might have the option to pay an additional fee and opt for the Priority Service (the service availability depends on your location). Your application will be at the front of the queue at every stage of the decision-making process
However, this option is not available to all applicants. Children who have been granted leave as dependants, but have now turned 18 years old, are ineligible to apply for an Extension from overseas.