How can I add a new function in options.c?

[acsglster <acsglster gmail com>]

I am trying to do some small changes to the options.c file in the fn-derivatives plugin directory (just for my own use): mainly just changing the default cost-of-carry to be equal to the risk-free rate rather than zero. I'd prefer to just add a new function with the changed default as opposed to modifying the existing function, but I cannot find any documentation on the steps to do this and it is not very obvious from the source. Any pointers? I am working with the "beyond linux from scratch" version of the sources.