Recently I've made the Pelican improvement mod (the speed brakes + lighting add-on). And it was extremely difficult to bring my 3D objects onto the existing Pelican by Asobo. I was stuck for days! Because of lack of documentation and lack of (or broken/outdated) tools.

While focusing on delivering the mod, I also noted for later that this was a task that maybe I could contribute in simplifying. Now it's the time and I've written some scripts for you (and also for me, of course).

So, I hope it will empower some of you into adding fun (or serious) stuff on existing aircraft and expending even more the richness of this community.

Disclaimer:

• Don't use these tools (or any other) to add stuff and republish aircraft 3D models of copyrighted/payware stuff. That would bring you legal problems!
• Even if you want to reuse freeware GTLFs and add a new set of wings or floats or a trailer attached behind the tail (that's the kind of stuff I could have done :D ), always ask the initial aircraft/models creators first: It's not true that "It's easier to ask for forgiveness than permission." 

Tools you will need:

• Blender 3.1.x or 3.2. AND Blender 2.93 (with add-ons that are listed on the PDF included with this mod)
• Python 3
• Probably MSFS Textures Toolbox (another add-on of mine)

Knowledge you will need:

• How to "use Blender": managing add-ons, import/export, modelling, animating, unwrapping, texturing, organizing your collections, etc.
• How to create MODs: folders naming and structure, generating/updating a layout.json, making your manifest.json, etc.

Behind the download button, you will find:

• Two Python3 scripts > MSFS-GLTF-Compiler.zip
• One PDF documentation > MSFS Gltf Merge DOC.zip
• One example MOD : "Toga Cola dispenser for the Cessna 208B" (see screenshots on this page) > lrdfrt-aircraft-208B-TogaCola.zip
• Example/Working files (Blender + PNG + various GLTFs) of the Toga Cola Mod > Toga-Cola Example files.zip

I will not put much more text here, everything you should learn is in the PDF (4 pages, a big [and complex] schema). And if it's not in there, maybe I forgot to explain it, or maybe it's something you should already know before stepping in this thing :)

Future updates:

Right now, you will see this process is utterly complicated and could really benefit from one or two little simplifications that would make the whole thing so much more easy and even automatable. So I will think, refine and see what I can do within the limitations of code copyright from all the tools involved in this mix.