I guess we can generate custum material parts just like ContentTweaker.
I tried to add an .toml file in config/jaopca/modules like thisbut it was useless.
Can you tell me how to or it's not possible.

There should be a custom.json in the jaopca folder
I haven't documented the format yet, but try putting this into the json file

[
    {
        "name": "singularity",
        "type": "item",
        "materialTypes": [],
        "defaultMaterialBlacklist: []
    }
]

I found custom_form.json and put

But it doesn't seem to create any items

Sorry, forgot to specify the values in materialTypes
Try replacing the materialTypes line with

"materialTypes": ["ingot", "gem", "crystal", "dust", "ingot_plain", "gem_plain"]

You can remove the material types you don't want

Thank you,I also want to know if I can add custom materialTypes?

There is no way to add custom material types and materials, and these are all the material types.
If you want custom materials you should probably use ContentTweaker because what JAOPCA does is generate items based on existing items.

OK