There is no need to create dedicated extension for VS 2013. Addin for VS2012 works perfectly fine with VS2013. All you have to do is to copy:
1. %user%\Documents\Visual Studio 2012\Addins\ into %user%\Documents\Visual Studio 2013\Addins
2. C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\V110\NaCl\ to C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\V120\NaCl\
3. From C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\V110\Platforms\ copy folders:
ARM
NaCl32
NaCl64
NaClARM
PNaCl
PPAPI
to C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\V120\Platforms\