When looking at the fireflow report (fireflow flextable), my user data extension (UDX) is not available in the list of attributes to add.
The fireflow report is a special table that combines both the Junction and Hydrant flextables. Since it’s possible to create a user data extension for one of these two element types and not the other, user data extensions are not currently supported.
This has been logged as Enhancement #257943
As a workaround:
1) Create a junction flextable
2) Add in all the fireflow related fields (all the ones in the fireflow report are available)
3) Add your user data extension.
4) If you also have hydrants as part of your fireflow nodes, create a hydrant flextable and do the same.