Create partial class into an flow machine

Hmmm... do you mean turn a partial class into a flow machine, access a partial class in a flow machine, or use a flow machine like it was a partial class?

Oh, they all look interesting. The best choice would be to use a flow machine like it was a partial class

Okay, If you're looking to reuse functionality across lots of different objects, then you'll want to look at Macros.  

For the partial class aspect, you're looking at super units.  You can contain a logical grouping within a graph, and then stitch those graphs together in another Flow Graph or in a State Graph (via flow states).  That should cover the functionality with only the type restrictions imposed by C# missing.