It is possible to edit the built-in snippets of Xcode as described here , but remember that changes may be lost when upgrading to a newer version of Xcode.
In particular, with Xcode 6.3.1 (6D1002) , the Core Data Fetch fragment can be found on line 681 in the file fragment:
/Applications/Xcode.app/Contents/Frameworks/IDEKit.framework/Versions/A/Resources/SystemCodeSnippets.codesnippets
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