As far as I know, there is no way to edit the complete forms themselves that ESRI provides, you can only access them and use them.
A solution for your problem involves two parts:
1) A form that that accesses the functionality of the IIdentifyDialog interface and has the customization you desire. You could make it very similar to the ESRI form by plugging in various properties of IIdentifyDialog but then add your own tweaks such as the listbox you mentioned.
2) A custom identify tool that instantiates a new IIdentifyDialog, but doesn't actually show the dialog. Instead, it shows your form. The OnClick method of the tool opens your custom form and then calls the methods and properties of IIdentifyDialog without actually showing the dialog and then populates your custom form. Essentially, the form would remain hidden in the background and do all the identifying work for you, and you could just grab the values of it from your instantiation of IIdentifyDialog and populate your custom form as needed.
The custom identify tool is already mostly written for you.
Custom identify tool in VB:http://edndoc.esri.com/arcobjects/9.0/samples/arcmap/identifydialog.htm
Custom identify tool in C#:http://forums.esri.com/Thread.asp?c=159&f=1707&t=172833