Is your feature request related to a problem? Please describe.
I would like to be able to add custom recipes to the Kiln, using CraftTweaker.

Describe why you think it fits in Ceramics
Because most big mods allow that kind of customization, and so should Ceramics

KubeJS is supported through custom data pack insertion e.g.

event.custom({ type: 'ceramics:kiln', ingredient: Item.of('minecraft:clay_ball').toJson(), result: Item.of('minecraft:brick').withCount(2).toResultJson(), experience: 0.3, cookingTime: 100 })

There is customization, using data packs. I would highly advise you learning them as they give you a lot more customization.

I have no plans to add crafttweaker support myself. If someone pull requested it I would look 8nt9 it, but I already offer datapack support. Crafttweaker even has a way to add JSON for recipes so you can add your own "support" via.l a function

What about KubeJS compatibility?

How is that different? I support datapacks. Anything that is not a datapack I will not write compat for, so only if someone else writes comap.