Hi Cristiano,
I'm afraid there's no easy way to do this... which is why only one workflow assistant currently exists. It was really awkward to create new ones, with various fiddly manual steps. I had considered removing the panel entirely, although it seemed quite useful and so it remains until there is a better way.
Anyhow, if you would like to have a look at how it works then you could search the repository for 'WorkflowPanel.java' to see some of the code. There are also various html files and images in the resources of 'qupath-gui-fx' which are needed to make it function properly. These were all created and arranged manually, I'm afraid.
It might be possible to add to what is there using an extension, but I suspect it may be easier and better to write something new.
Pete