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 ?
iOS6 simulator is available in Xcode5
No.
Apple only support iOS7 and iOS8, and they're strict with that. As a result, only iOS7/8 simulators are available in Xcode 6.
If you want to run on an older simulator, you need to develop on an older version of XCode.
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!
Not possible. But: you can still test on a iOS 6 device from Xcode 6. I keep an old iPhone 4 for this (look at eBay).
EDIT: there is another way - install Xcode 5 on a virtual machine (e.g. VMWare Fusion) running Mavericks.