add elements to existing standard structure

This comes under the heading of "I'd like to know."

Suppose that I have a function that uses struct tm, but I want to add add some additional elements to that structure.

I know for instance that I can do something like
struct foo{
    struct tm mytime;
    int xx
}

I know that I can address elements of tm within foo by for instance, foo.mytime.tm.year, but is there a way to create a structure foo that inherits the elements of tm?  I've never seen it done, so if it is possible, is it reasonable.

Thanks