You can edit Relic .rgo files but the trick is you need a custom .rgm file with the same name as the .rgo you are trying to edit. It's been so long so I don't remember everything, but that might get you closer.
Hmm, I tried using custom .rgms and changing its name to be the same as the one I want to edit, but it turns into an abyss. Like as I open it, the original model of the custom rgm appears there instead with a broken cluster of its vehicle states. Someone used to upload an empty .rgm file on the old forums just for that, but the link for it is broken
If I can find that guy, whoever it was, maybe just by chance he/she still has that file somewhere.
Hi
Although I'm not a Senior Modder I did find old documentation for the Object Editor that might be able to help you.
Community Object Editor Documentation
I found this the same day I posted this question actually, sadly it only shows how to open a custom model with the rgm trick.
I really appreciate the help from the both of you, I've been doing CoH 1 modding for over a year on my own and during my own free time.