Is there a way to assign categories to portfolio-items from within the codeless-builder? I know it is straightforward to assign categories from within the main WordPress-editor.
Sorry but that is not possible. You can check out the available options you can customize for a portfolio item within the builder, by clicking on the little icon at the right-bottom of the portfolio item. See screenshot: https://postimg.org/image/en7cec56d/
Let us know if this is what you where referring to.