The basic trick to making a multi-page form is to have one variable that will contain all the data, then have several different pages, each with its own Data Page builder, all pointing at the same variable. Then, add a Data Field Modifier to each page, hiding all the fields that you don't want on that page. When Page Automation makes its SaveData method, it knows only to affect the fields that are actually on that page, and it leaves anything else alone. So the successive SaveData methods, one for each page, will each leave the data set by the other pages alone. All you need to do it make sure that each field appears on exactly one page.
Then add navigation between the pages, and only do the actual work in the submit action of the very last page.
The following article on developerWorks describes how to build a multi page form wizard with WebSphere Portlet Factory in more detail:http://www.ibm.com/developerworks/websphere/library/techarticles/0909_rekaby/0909_rekaby.html