Hello, 

After further research this is considered to be a new Product Idea. At the moment instantiating fully functional igTree in igCombo is not supported.
I would recommend you to log a feature request at http://ideas.infragistics.com and powered by UserVoice, the Product Ideas site allows you to suggest new product features, new functionalities in existing products and controls, and even request new controls and products altogether. Members of the Infragistics Community then vote for the features and suggestions they want to see added to the products, resulting in the most popular features bubbling up to the top. When planning new releases, our Product Management team will look at the most popular features and use your feedback to prioritize upcoming work.  

Currently I am trying to create a sample using custom approach, and yet since the igCombo is not designed to operate with hierarchical data, it could prove quite difficult.
I will update you in case of success. 

Thank you for using Infragistics forums!