marferit,
The most common way to handle this would be to specify devices at the locations where you are attempting to measure output.
If you need output as represented by slices, boundary files, or plot3d files, the best way to convert the output to numerical data is to use a tool provided by the team at NIST.
If you go to your PyroSim install folder, and then the fds folder within, there will be a tool called fds2ascii. While we don't explicitly provide support on how to use it, the FDS User guide provides some documentation on the tool.
Hope that helps!
Regards,
Jon Albrecht
Thunderhead Engineering