It is possible, with a good deal of work. There are some programs floating around on the forums that let you convert the zone files to .obj files you can work on with something like Blender. You then convert them back and load them up. As The_Beast said above though, any zone file changes are client based; meaning you need to distribute those files to your players or they will not see the changes.
The client itself is also hardcoded to load specific zone names, so if you want a completely custom zone you need to replace one of the existing zones that you do not want to use. Not really a huge issue as there are so many, just something to be aware of.
Here is a video I made of me running around in a map from Ultima Underworld which I imported.
https://www.youtube.com/watch?v=0r6s6x6hVL0&t
Also, provided you are familiar with the code base you can certainly do some heavy heavy modifications. There are other videos on my channel showing me trading with NPC's (not just buying and selling, but actually comparing and trading items) and testing out a required stat system (ie, you require 100 STR to wield this axe kind of thing). Provided you can work out how to deal with the limitations of the client itself, you can do pretty much whatever you want.