What technology is your application using? If it's just a Windows app or a WPF app, then it may be possible to create a UI Connector or Type Extension, where you can write your own code to get the functionality you need for your Treeview control. You can check if you can create the UI Connector by right-clicking on the interrogated control and checking if the UI Connector option is available. Here are some documents to help you get started with UI Connectors:
Alternatively, you may be able to reach the next column by using Methods that contain the "Path" parameter. For example, you can manually select the cell that you want to add text to, then, using Test Methods, you can call the getSelectedNode (or similar methods) which should return the path of that cell if it is supported. You can then use this path on methods containing the "Path" parameter.