Yes, some of it is possible, however it's far from easy. You can't just "open, modify and save" an Age because they are already pre-compiled. Modifying an Age is a lot harder than creating a new one from scratch, so I'd advise training on creating a new Age first.
The reason it's so complicated is that you have to import the Ages into Blender first. The importers ignore a lot of data when importing Ages, which means you can't export the Age again, since it would break all animations and clickables, as well as make the Age extremely ugly. Because of this, you have to use annoying workarounds like exporting only the objects you edited, then load them after
the original Age was loaded. Merging two or more existing Ages is even more annoying.
Usually the best way is to do like Cyan did with the Great Zero: create a new Age that looks a lot like D'ni, and create your new area there. This is what is done in lots of Fan-Ages that are supposedly in D'ni, like the Messenger's pub and Fehnir's House.
Either way, you'll need Blender
to create the locations, and Korman
to export the content you created.
If you really want to have a look at Cyan's Ages, you can use the PyPRP importer
(for Blender 2.49) or my own importer
(for more recent versions of Blender) to import the Ages (obviously I'll recommend my own importer as it's more recent and runs on a newer Blender). Again, these importers only import meshes and some textures, but break everything else, so don't expect to be able to re-export those areas.