OH3: How to extend the semantic classes

Not without contributing some code or suggesting them in an issue in GitHub · Where software is built, I’m afraid…

They’re defined here: openhab-core/bundles/org.openhab.core.semantics/model/SemanticTags.csv at main · openhab/openhab-core · GitHub
and translations are here: openhab-core/bundles/org.openhab.core.semantics/src/main/resources at main · openhab/openhab-core · GitHub

If you don’t find the class you need you can use one more generic (like Equipment, Point, Room or Floor), with some synonyms it could be enough in most cases.