Foreign Function Interface (FFI) for RPN calculations

Hi there,

I am new to RRDtool, but it looks very promising for monitoring my
measurement task. However, I need to postprocess some measured values
with a fairly complex algorithm which is currently implemented in a
legacy Fortran program.
Is it possible to call external functions (C or Fortran) from within
the RPN declaration? I could not find an example and I fear it is
beyond the design and purpose of RRDtool. 
What are best practices for such use case?

Thanks in advance,
Martin (smartmic)