Hey, does the plugin has support for itemsadder or any plugin like it? If not, are you planning to add it in the future? Thx

already has custommodeldata support

Okay so! The latest snapshot should implement support for this as you can now save custom items from other plugins like ItemsAdder into ItemJoin via the /ij menu (click the hopper). NBT tags are now dynamically fetched and saved to the properties option for ItemJoin which allows the tags to be added when the ItemJoin item is given, retaining the functionality from the other plugin as well.

Link: https://www.spigotmc.org/resources/itemjoin.12661/download?version=577922
Remember to remove your old ItemJoin.jar file

If this doesn't function as expected feel free to re-open this feature request.
Thank you for your patience!