I'm interested in this too.
If it's not possible, what should I modify in jboss portal code to make it possible?

Problem solved by modifying portal source :)

you don't have to modify portal src!
The correct way to appoach this is the layout strategy. Take a look at the MaximizingStrategy as an example. In the strategy you get all the portal objects and their navigational state and you have the ability to change their state or position on the page, you can filter portlets, etc.
If this doesn't let you do what you need, there is a design problem in the strategy!

From what i've seen in the sources, when a portal page is visited for the first time all portlet windows are assigned new, default navigational states (this being WindowState.NORMAL, Mode.VIEW).
I wanted to create a portlet window initially maximized based on a property taken from configuration. At layout strategy level i didn't find any way to access properties defined in the configuration (but I didn't look for it too long).

Perhaps you could get what you wnat by using several strategies?
The strategy has no access to config info, that is correct. But you could configure different strategies for different layouts, or pages. Not perfect, but perhaps enough?

Where is your configuration, and what does it look like?

you can use ctrl:windowstate=maximized in url .
give a url for example:http://jsports.org/portal/index.html?ctrl:cmd=render&ctrl:windowstate=maximized&ctrl:window=default.forums.ForumsPortletWindow&op=showTopic&t=59

or http://jsports.org/portal/portal/default/articles%20and%20news