Possible to inject new menus into GUIs?

[chants]

Yes as Kurapica points out I assumed Windows desktop API native apps as in link from atom0s. If its Delphi, .Net, Java or VB6 or the like different approaches are needed based on their interpreter. Further in UWP apps though there is no menu, such modifications are much harder due to the maze of security restrictions and limited API you will have access to, making it probably the hardest of all.

Also if it is a multithreaded app, accessing data structures may require sharing synchronization objects of the app.

Generally a task like this will be as much of a careful programming effort as it is a reversing one if you want a clean reliable solution. Adding functionality is significantly harder than modifying or removing it as a rule of thumb.