Hi Armando,
You will either get the ALBaseApp in your ON-Premise installation folder or you will get it from Sandbox.
All you need to do if figure out the Report Backend Name and ID using Inspect Ctrl+Alt+F1 on Request Page, and using Report ID or Name search in ALBaseApp.
Further you need to change Report Name and ID. This will include a single .AL file and .RDL file.
At this point a copy of report is ready.
Now you include that report on your Extension App and check the DataItemsfor it and add your extended field.
Build you App.
After this, you will find the field in RDL Datasets and there you can add it.
You can refer to one of my blogs www.cloudfronts.com/.../
Hope this helps