Not that I found. (Also switched from VBS to C# Script recently)
However. If your existing stuff is a framework, and NOT test specific, you could use it as script extensions and integrate them directly into the IDE. That's what I've done. My framework is written in VBS and added as script extensions. My current project is using C# Script, but still uses my VBS framework as it's now part of the TestComplete IDE.
Info on script extensions/plugins: http://support.smartbear.com/viewarticle/69987/
(The above assumes they are happy for the framework to remain as is and only tests themselves to be C# script. Once in place, the framework shouldn't really change much - mine doesn't. If they are insisting that your framework also be C# Script for maintenance purposes, the abpve won't help.)
