Adding a new menu to the player menu

[Awaz]

Just a brief background, I have been involved in IT for the past 14 years. I did a lot of coding in archaic languages as well as C#, C and JAVA. Used DB2 as database, yada, yada. Although, for the past 5 years or so, I am doing more management and lead type stuff, so not much coding. Now, I have never coded in XML, but I have to review and understand XML codes written in projects I manage.
Trying to get back into technical side of things (personal level) and been trying out some Lua code. Got this game not too long back and would love to try out some addon. The first thing I want to try is to see if I can add another menu to the existing player menu that comes up when you press the ALT button. I tried to look around to see if there is a pre-defined XML that lays out that UI, but I did not find any. Is that even possible? If so, any direction on how I go about it?