It may be possible to put inline C# inside an Xslt template but probalbly not worth the hassle.
1. You can use a custom Xslt file with the mapper and add inline C# as described by Nick Heppleston above.
2. You can also add a scripting functoid with your C# code that is not connected to anything and call this from your Xslt template functoid.
3. You can also create an external library and call this from with your Xslt template (need to add an external assembly scripting functoid) You can also call this external assembly in a custom Xslt file by adding an Custom Extensions Xml file to your map definition.
The easiest way to discover how to do this, is to create a map using various scripting functoids, use the Validate Map command and view the resulting Xslt.