How to extend an existing PC board?

I want or need to add some simple parts, like level shifters and resistors, to Raspberry Pi 4 IO board. First, How can I extend or modify an existing outline of a board. And of course later extend its layers.
There are a lot of other questions, but lets start from this.

The simplest way is to just copy the project into a new directory, and then start modifying it.

The PCB outline are just regular graphic lines on the Edge.Cuts layer.

I see that now the Raspberry project package zip contains strange file names, like CM4IOv5.kicad_pro. But Windows wants files with ( names with a dot). I wonder how Windows 10 could open them yesterday but now I use W7.

Edit: Win10 seems to be happy with those Unix like filenames. Too bad W7 is not supported.

I could modify some lines. Lets hope they were the boards outline. There are so many similar lines in the drawing that it is hard to see which is which. Different colors could help.

The Layers Manager on the right panel has a layer called Edge.Cuts; that is the board outline layer. In my instance of KiCad, I made the line color red.

Alternatively, one can check the board outline with KiCad’s 3D Viewer:

ON EDIT: Ooopps… should be this color:

Thank you. A new CAD means new names too.