Do you have a coding example?

       ILoadOperation loadOp = IOperationFactory.instance.getLoadOperation(new LoadDilemmaHandler() {
           // override any dilemmas where you want more specific input from the user
       });
      ISharingManager sharingManager = FileSystemCore.getSharingManager();
       loadOp.requestLoad(sandbox, relativeLocation, workspaceConnection, component, versionables);
       loadOp.run(monitor);
This is the basic part of getting a load operation and running it. You would have to write the part of getting the versionables that you want to load. You can use the sharing manager to create and register the a sandbox. The relative location is relative to the sandbox and can be null to load in the sandbox root.

A lot of what you want to do here would be reimplementing internal code but I don't know how complex or simple you want this implementation to be. What is the reason that you don't want to write a script using the command line?

You could follow https://rsjazz.wordpress.com/2013/02/28/setting-up-rational-team-concert-for-api-development/ to set up an API development environment and then follow https://rsjazz.wordpress.com/2013/03/20/understanding-and-using-the-rtc-java-client-api/ and just use JavaSearch to find examples in the SDK.

 After loading you should also deregister the sandbox right?

I am certain there are different answers dependent on what you want to do.