Using System fonts into processing is quite simple as seen in the examples below. This approach dynamically loads the and displays the font, using vectors information, but be aware that this might not work for all fonts and appearance may very from computer to computer.
PFont font;
size(200, 200);
String[] fontList = PFont.list();
printArray(fontList); // print out a list of all system fonts available.
font = createFont("Trebuchet-BoldItalic", 50);
textFont(font);
text("word", 10, 50);
Font can be saved within the sketch folder to, incase the font isn't installed on the computer running the sketch.
PFont font;
// the .ttf file should be located in the sketch folder
font = createFont("comic-sans.ttf", 32);
textFont(font);
text("I (heart) Helvetica", 10, 50);