Populating a va_list

Is there a way to create a va_list from scratch? I'm trying to call a function that takes a va_list as a parameter:

func(void **entry, int num_args, va_list args, char *key); 

...from a function that doesn't take a variable number of arguments. The only way I can think of is to create an intermediary function that takes varargs and then passing along its va_list, which is pretty stupid:

void stupid_func(void **entry, char *key, int num_args, ...) {
    va_list args;
    va_start(args, num_args);

    func(entry, num_args, args, key);

    va_end(args);
}

Is there a better way? I can't change func's signature.