They could add the option to set the arm part of the java model armatures, as there is currently no such function, so even if I model the arm part, I can't make them move.
Issue comments
im testing on 2020.4 but no luck; i used 2 types of models (blockbench); on the first i made 3 separeted groups (armor, armL and armR) and on the other i put all on the same group (2 arms groups inside the armor group), i imagine the first model is the right yet its not working, i put all the UV box inside the 'Body' space
You can unlock them in 2020.4 after this is added. Also you have all explained here: https://mcreator.net/wiki/locking-code
https://mcreator.net/tracker/issue/63973
Check changelog, you will see this fix is coming in the upcoming snapshot..
Please read the website a bit.
I could add them as an optional part of the chestplate.