Hi,
Unfortunately it's not possible to create Add-ins based on control beyond the ones currently supported in 1500 (button, gallery, dockwindow, extension, combobox, checkbox). We'll bear in mind your request for future releases. I also recommend submitting your suggestion at http://ideas.arcgis.com.
Re the Analysis drop down window - this is a Gallery Add-in, although it's an internal extension of it which allows additional menu items at the bottom. However - you can get quite a lot of flexibility in the Gallery by implementing different behaviour based on what gets clicked, adding other images/text as separators. See this Add-in for an example: http://www.arcgis.com/home/item.html?id=b2814df318ad4b3482cf05ce62db1967.
Cheers
Mike