Seba,
How to do screen modifications starts with you downloading the screen editor.
I gave you the link. Then view / study the tutorials and videos on that site.
I assume you are using the standard Mach3 mill screen which is 1024.set.
Make a copy of that file and place it in a safe place.
Now you must go through a learning curve,
Open the mill screen using MachScreen.
Click the Help Tab and review all the help.
Play around with all the controls, etc, but don't save anything when you exit MachScreen.
I suggest you download and review the Mach Customization Guide and Macro Programmers reference Manual.
Ask questions after you do the above.
Yes, someone can do what you want quickly ,BUT, it's better that you learn.
Have some fun with the learning curve ,
RICH