Can a select drop down list be added to my winJS toolbar via Javascript?
Try using "content" command type. As per the documentation @ https://msdn.microsoft.com/en-in/library/windows/apps/dn904220.aspxits supposed to support <input> tag.
This creates an Command that can host other HTML markup inside of it, including text, <input> tags, and even a subset of WinJS controls. Only a <div> element can host a content Command.
UPDATE
https://jsfiddle.net/vnathalye/yg0rs4xc/
You need to create a <div> tag and pass it as first param to new WinJS.UI.Command.
Once that is done you can add select drop down or any other control to that div and that should appear in the toolbar. In the above jsfiddle link I've hardcoded select tag in the div.