Which is the additional stroke added to 末 and to 未? significance?
末 is logographically comparable to 本. The top stroke indicates the top branches of a tree, the end of the tree. Hence the meaning "end".

However, 未 is unrelated to both characters above, it is just a representation of a tree. It is one of the twelve earthly branches, and as such a character that once had some original meaning that is now lost to us; it has been abstracted to mean "not yet".