Can't build Metal Bars on Any Floor but pregenerated

You can’t build metal bars on ANY flooring other than “floor”, which is pregenerated in houses. No buildable floor counts. That’s ridiculous, it should include ANY floor that’s not just dirt.

Feel free to contribute to construction recipes.

Is there an official way to contribute if I go learn how and produce some?

Yes. The official way to contribute is done through github. You can find more information in the contribution guidelines.