I'm working with Enterprise Architect. I would like to see simultaneously the class diagrams from all the packages in order to better visualize the relationships between classes. Is it possible?
No such thing exists, as far as I know. Your options are to draw a new class diagram containing elements from the existing diagrams or create a package diagram which should describe the relationships between different packages. There is a reason for that - relationships between classes in different packages are on another level compared to inter-package relationships and normally they shouldn't be mixed.