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 ?
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.