[Therion] Include PDF or SVG graphics in METAPOST macro

Hi, Is it possible to include PDF or SVG files in METAPOST Macros? I want to define complicate symbols like trees to therion. It would be easy if I can include a PDF o SVG file and define the scale in a METAPOST macro. If not possible, do you know a software to translate a PDF or SVG to Metapost macro. Thanks, Evaristo.