One way to do it is through adding a variable {$variable} to the dialogue, and C# can manipulate that variable.
If you want to do more, you can also use the conversation. Since it has you can put character portrait left etc: dialogue, you can attach that to a string as well. Right next to the text box when you put the conversation in, there is a drop-down box where you can assign it to the string variable you have in any flowchart. As long as you create an object Flowchart flowchart (or any name you want really)..you can get and set the string variables and manipulate it.:) The way I am doing it now (not clean enough for an extension), I assigned a script. Then I use the invoke method, attach the game object I assigned the script to as a component, and make the calls.