There are three fonts in Toribash and I don't think you can add any. It works like this:
draw_text("FONTS.BIG = 0", 5, 100, FONTS.BIG)
draw_text("FONTS.SMALL = 1", 5, 155, FONTS.SMALL)
draw_text("FONTS.MEDIUM = 2", 5, 170, FONTS.MEDIUM)
and looks like this:
You could use the number values directly like this:
draw_text("FONTS.BIG = 0", 5, 100, 0)
draw_text("FONTS.SMALL = 1", 5, 155, 1)
draw_text("FONTS.MEDIUM = 2", 5, 170, 2)
But I think you can read the code better if you use the names.
Also you can't change font size.
To use your own font you could create image textures for every character and then use load_texture() and draw_quad() to draw the text. I'm not sure but I don't think you can resize those either.
If you want a resizeable font you could create your own vector-like font by using draw_quad(), draw_triangle(), etc. to create letters and then scale the coordinates.