It depends in what way you need to edit the bus.
The configuration files that tell OMSI what model parts to load and how to animate them, etc. are all plain text files and can be edited easily. If you simply need to include a model file from one bus in another, it's not too hard to do. However, if you need to edit the model itself to move/rotate it, or to edit models in the destination bus to accept this component, then you'll need to acquire the original model files for the buses from their respective authors, and get permission to make and/or release your modification. Converting the o3d files (which are encrypted) into a format that can be read and modified by another tool is in breach of the copy-protection put on the files.