Making a addon for a mod that was not made using mcreator

You can use ore dictionary to link with other mods and NBT tags. You can then list other mods as dependencies in workspace settings.

For more advanced things and APIs, you need to code.

Ore dictionary is a godsend, essentially, if another mod makes a ruby ore and I make a ruby ore, If we both use ore dictionary then they will be considered the same ore
As to how to use it, it is one of the settings during quite a few mod element creation processes

If you want a better example, ore dict. is like how all the planks can be used to craft sticks and such

@klemen do we need to import the mod