If you have the actual code you can use, you can just paste it in. Or put it in a separate tab if you want to keep things neat. If you are actually using a library (which is not code!) you'll need to install the library in the libraries folder within your processing directory, and then add it via Sketch > Import Library... > [library name], which should add a line like "import [libraryname].*;" to your code.

I didn't knew the "tab" story. I did it and it is OK !

But I don't know how to do a library.

I will just make a hard or soft link in any place that I want to use that shared code.

No way to share common code in Processing, unless you make a real library out of it, packaging it in a jar file.
Now, perhaps the hard / soft link can be an idea, to spread a .pde or .java file over several sketches while keeping them consistent.