I noticed that there are no preview or simulator for iOS6 in Xcode 6-beta, so I was wondering is there any way that I could add it manually ?
No, the earliest SDK you can get for Xcode 6 is 7.1. By default, the iOS 7 simulator is not installed but you can install it by going to preferences, then to downloads and installing it from there.
To run iOS 6 applications, try googling around for an earlier Xcode version (late versions of 4) and these should have the simulator!
Hope that helps!