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.