The feature is available as of Test Studio Ultimate R2 2018 (v.2018.2.530).
The implemented integration between Test Studio Web&Desktop and Test Studio for APIs projects allows you to insert an API test and execute it as a step in a web test.
To add a new API test as step first create a web test. Then the option is available in the Step Builder among the steps in the Common section.
Double click it to trigger the 'Select API test' dialog.
The available options are either to create a new test or import an existing one.