I haven't found a drawbars user interface like in the Garageband app. However, you can adjust the organ sounds by using the sound generator in Garageband for Mac. In the window where you select a software instrument, select one of the organs. Then, click the 'Edit' tab. Next, double click the 'Sound Generator' window. The 'Tonewheel Organ' interface will display. There you will find various sliders including one for 'Drawbars'. You can also adjust the Percussion level, etc.