You could check out the source for the GPD which is what I use in Eclipse to define my jPDL and it automatically creates a png copy of it.
It does rely on the x,y,w,h co-ordinates though which probably will be the hardest bit.
Unless the processes are simple a->b->c->d then I would investigate employing a diagram layout algorithm. Check Google for some answers.
You could also use alternate tools to generate a BMPN diagram and then import it but I don't know if that would fit with your project.
James