Yes, you can install c# 6.0 into VS2012 and VS2013 on a per project basis as a NuGet package. You’ll have to install this package for every project that you want c# 6.0 features in.
https://www.nuget.org/packages/Microsoft.Net.Compilers/
Installing Latest C# Compiler via Nuget
Install-Package Microsoft.Net.Compilers
EDIT: As pointed out in the comments below, upgrade your NuGet to the latest version in order to ensure full compatibility with the nuspec file.