are this legal in mylot to add someone as friend without respond first !

There aren't rules about who you can or can't request as a friend. So yes you can request someone you've never responded to. But friendships here are better if you have something in common. So if you can't respond to any of their discussions, why would you want to be their friend? When I get a friend request I usually check what kind of posts that person makes and use those to decide if I want to add that person or not. If they have responded to me it's more likely that I will accept them.

There's no problem to propose friendship without first having read or answered discussions (some people, for example, don't do discussions, just hang pictures and leave personal comments). Although when somebody has to decide if accepting your request it's usual to look at what have you to say about some subject, and if it's one they started, they will tend to be better suited to give your opinion its due value. So it is not mandatory, by far... but it helps :)