Couple of different types of add-ons
Surface Bases
-easiest to start with, just need to write a config file & maybe add in a few custom meshes
UMMU/UCGO specific addons
-Also very simple, but might require a bit better modelling skills
Custom Vessel
-Harder, but depends on which route you take. If you want to write something that works quickly, use spacecraft3, but its very old, and likely will be broken soon with new features in the latest Orbiter Beta. Other options could be writing a dll, but that will take a fair amount of time to learn properly
I would highly reccomend the first option. Learn how to model using Blender, and create a nice little surface base