So, just add them the way you do.
Or maybe you want to sort a list with the selected routine, something like:
void Custom_container::add_sort(bool (*ls)(T,T))
activeSort = ls;
which can call sort with activeSort as a parameter for the given list you want. Except if I am missing something...
void Custom_container::select_sort(int index)
activeSort = sort_routines[index];