I am not a savior, but adapting the substuck system by extracting the one that comes with l2jmobius c6 interlude took me only 10 minutes and it was not difficult, I have to do a lot of tests and verify if it does not give errors or bugs, but so far it works.
There is the code with the modification. Use it at your own risk, if there is an error or bug, you will have to try to solve it, you can still talk to me and I will help you if I can, otherwise you will have to seek external help.
https://pastebin.com/YxKiDvS0Just so you know, this code enables the substuck system, but still maintains the restrictions:
warsmith & overlord only for main class, using them as a sub is not allowed.
kamael only for kamael, they cannot be mixed with other classes.
Also all passive skills stack, including robe/light/heavy mastery, there is no restriction or any protection against this.
I was forgetting, you need to create the SubStuck.ini file in game/config/Custom
in which it should be like this:
# ---------------------------------------------------------------------------
# SubStuck
# ---------------------------------------------------------------------------
# When True, this will allow a person who changes subclasses to keep the skills of the previous class.
# When False, this will remove all skills of the previous subclass and only award skills of the current class.
# Retail: False
KeepSubClassSkills = True