What is the best way to set: input type hidden in JDOM. I want to hide some existing field. I am stil not good friend with JDOM

Change the namespace for html.form.input.hidden, and leave all the parameters as they are. Only the requested ones will be used.

Regarding JDom, I understand.
Coming back from the Joomla Day, I was definitively convinced about JDom, because Joomla still does not support fields as I like.

BUT in a close future, it will be possible to generate using JForms.