IDE additional extensions

In short, yes. There are different possibilities. Could more define what you exactly mean by extension or what you like to achieve?

The current most useful way is to create an own library in a project, what results in an own node representation in the left menu bar. Beware that it is project specific and not automatically available for all. These are programmed in JavaScript. Examples can be found on the external page https://makecode.com/playground. 

The mechanism to add global extensions directly from GitHub to the IDE is currently disabled because of security constrains. This is used for extensions shown via the Extension node in the left menu or Settings -> Extensions.