Yes, it’s possible. No special API key required - depending on space security settings, you have to either provide the space access password, or perform the request under a Windows account that is allowed to access the space.
See this post on how to access EasyMorph Server API from other applications: How external applications can run EasyMorph workflows
The post is about running tasks, but uploading/downloading files to/from EasyMorph Server is equally possible in a similar fashion.