I see.
Then you would first have to have (make) a server that serves up *.st source files throug a REST api. Then you would modify the ./Playground project to fetch the files from your server in stead of from the TextArea. This is done in the PlaygroundApp.evaluate method.
Sure, you could do that too in SmallJS.
See the classes in ./Smalltalk/Core/Fetch and in ./Smalltalk/Node/Server.
The Fetch unit tests actually have a case for this, I think.