If I understand you correctly you want a treepanel inside a combobox?
If so, you can use this ux: http://docs.sencha.com/extjs/4.2.2/#!/api/Ext.ux.TreePicker
In SA you have to
- set the userAlias (xtype) to treepicker.
- copy the source code from TreePicker to ext/ux/TreePicker.js