Can you add a letter to a hoizontal word and make a vertical word from that in one turn?

Yes. For example if the Word "LIGHT" was on the board horizontally and you added an S to the end of it, while also making the Word "SCOPE" vertically you would get points for both Words. It's a great way to make lots of points!
Yes you can. If you attach a Word to another Word, you can score both Words in the same turn.