Depends on the build you're on. Microsoft had removed the feature of manually loading extensions in the latest RS builds, probably since 14342. So, if you're on one of the older builds then
this WinC article will help you. Else, you can use your Outlook email address to download apps and extensions from the Store by signing in to install each app or extension. (FYI, this could be done on Windows 8.1, I've not tried on Windows 10. But you may try it). Anyways, you cannot sideload extensions on newer RS builds.