Secondary variable mapping

I would like to map “permeability” as a secondary variable for the implementation of Kozeny Carman equation. Can anyone please help me on this?

#include “SecondaryVariable.h”

namespace ProcessLib
void SecondaryVariableCollection::addNameMapping(
std::string const& internal_name, std::string const& external_name)
// TODO check for missing secondary vars.
** // TODO check primary vars, too**
_map_external_to_internal, external_name, internal_name,
“Secondary variable names must be unique.”);

Thanks in advance.

Harish R