We are allowed to add fast tabs, fields and actions to existing pages. The actions can be connected to code unit objects using an action's RunObject property.
The MSDN entry, Extension Packages Capability Support Matrix, describes what changes can be made to Table and Page objects to support an Extension (https://msdn.microsoft.com/en-us/library/mt574414(v=nav.90).aspx).
In the next version of NAV, Table Changes and Page Changes will be replaced by new object types called Table Extensions and Page Extensions. You can read about these planned NAV enhancements on MSDN at https://msdn.microsoft.com/en-us/dynamics-nav/newdev-table-ext-object.