可以.
先说who,
主语从句：Who killed the man is still a mystery(谜团).
这是一个主语从句.
同位语从句：The question who will take his place is still not clear.（who作主语）,是主语的同位语.
再说how,
主语从句：How he killed the man is still a mystery(谜团).
这是一个主语从句.
同位语从句：The question how he killed the man is still not clear.（who作主语）,是主语的同位语.