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.
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.
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.
iOS6 simulator is available in Xcode5
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!
来源：https://stackoverflow.com/questions/24038286/is-it-possible-to-install-ios6-sdk-on-xcode-6-beta