Can do this, But not really necessary
How to do this
Jason Lee got the answer. When installing xCode I preferred keeping previous installations rather than replacing them. So I have these in my installation Folder
So /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs
Contain different sdks. (Replace Xcode.app with correct number) copy previous sdks to
/Applications/Xcode 3.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs
Here is my folder after I copied one.
Now restart xCode and you can set previous versions of sdks as base sdk.
Why it is not necessary
Refering Apple Documentaion
To use a particular SDK for an Xcode project, make two selections in your project’s build settings.
Choose a deployment target.
This identifies the earliest OS version on which your software can run.
Choose a base SDK
Your software can use features available in OS versions up to and including the one corresponding to the base SDK. By default , Xcode sets this to the newest OS supported by Xcode.
Rule is Use latest as base SDK and set deployment target to the minimum version app supposed to run
For example you can use iOS 7 as base sdk and set iOS 6 as deployment target. Run on iOS 6 simulator to test how it works on iOS 6. Install simulator if not available with list of simulators.
Additionaly You can unconditionally use features upto iOS 6. And Conditionally you can support new features of iOS 7 for new updated devices while supporting previous versions.
This can be done using Weakly Linked Classes ,Weakly Linked Methods, Functions, and Symbols
https://developer.apple.com/library/ios/documentation/developertools/conceptual/cross_development/Using/using.html#//apple_ref/doc/uid/20002000-SW3
Weak Linking
Suppose in Xcode you set the deployment target (minimum required version) to iOS6 and the target SDK (maximum allowed version) to iOS7. During compilation, the compiler would weakly link any interfaces that were introduced in iOS7 while strongly linking earlier interfaces. This would allow your application to continue running on iOS6 but still take advantage of newer features when they are available.