It is theoretically possible to replace the NUnit binaries that NCrunch uses (which can be found under Program Files (x86)\Microsoft Visual Studio 1X.0\Common7\IDE\Extensions\Remco Software\NCrunch for Visual Studio 2010), although the results of this will depend entirely upon how compatible the version of NUnit is with NCrunch's NUnit adapter.
It's quite unusual to do something like this, as most versions of NUnit are backwards compatible, and NCrunch 1.45 is packaged and integrated with the latest version of NUnit (2.6.2). Which version of NUnit do you need to run?