If you mean calling a FlowGraph “Custom Function” from within another FlowGraph, it is possible, but it is not very straightforward (something I want to improve and make far more convenient). To do this, please:
Drag and Drop the GameObject which has the target FlowGraph with the Custom Function you want to call, into the editor of the FlowGraph you want to call it from.
From the menu that will pop as usual by doing such a drag & drop, please search for “Call Function” node and add it (it is a method of the FlowScriptController class).
Make the newly created node “Callable” if you want to, as well as check the option for “Expose Parameters” to make this easier.
Make sure you set the “Name” of the function to be the same as the one you want to call.
Make sure that the “Args” you pass to the node are the same number and in same order (type-wise) as the function you want to call.
It’s easier that it may appear, but still not as convenient of course. I plan to make all this far more easier with a new “Call External Function” (or similar named) node however.
Let me know if that is indeed what you meant please.