Can I write configuration changes directly into XoomXML?

Technically, yes you can, though we don't recommend this as best practice.

Unless you're 100% certain of what you are doing, writing configuration changes directly into XoomXML is potentially risky because you will not get internal validation. We recommend users configure their systems using proprietary administration tools. These provide the internal validation that Xoom can not. We suggest Xoom is reserved for configuration management rather than configuration authoring.