Yeah. U can write the test script when u have only APK file. U didn't need Source Project.
U can use android studio and robotium recorder.
Robotium have only 5 free trial in android studio. So used it efficiently. Using Robotium recorder u can created the testing project for that APK file in Android Studio.
Then using UIAutomatorViewer (~/sdk/tools) which helps to find id, index, parameter details.
U can contact me any time if u any doubts regarding this.Because I have faced lot of problem while creating this test project.