You can design a multimeter in Linux with the help of a PCB CAD application, but you will have to know the schematics of a real life multimeter device since there isn't one pre-built. For this purpose, I would have recommended OrCAD, but it's not available for Linux. Instead, use KiCad EDA Software Suite. Access the official page and click on the Installation guide link on the right menu to install it first. Then use the tutorials and documentation to learn how to work with it and you will be able to convert your schematics for the multimeter into a digital one.