Yes, you can add an element from a list to this list, even without knowing its type.

GenericsFun.java

import java.util.ArrayList;
import java.util.List;

public class GenericsFun {
    public static void main(String[] args) {
        List<B> bs = new ArrayList<>();
        bs.add(new B());
        List<? extends A> mix = bs;
        appendFirstElementToList(mix);
        System.out.println("mix.size() = " + mix.size());
    }

    private static <T> void appendFirstElementToList(List<T> list) {
        list.add(list.get(0));
    }

    private static class A {

    }

    private static class B extends A {

    }
}