creating/accessing new runtime parameters

I am writing a backend C function whose behavior should depend on the
setting of a new runtime parameter that can be set with SET. I have
several questions concerning how to implement this, as I can find no
information in the documentation.

- How can I introduce a new variable (e.g., XXX) to the system so that
for example SET XXX=1 will work?

- How can I access the value of such a variable from within a backend
C function?

Thanks for your help.

Cheers,
Brook