Hi @ramon_pose,
In short; That's not possible. A single element can only have a single IFC class
Some additional info:
Only the top-categories can be set to (supported) IFC Classes.
The subcategories can be used to exclude some geometry from families.
On top of that (as override), each element in Revit (by type or instance) can be assigned to a different IFC Class than it's category (by setting it's IfcExportas parameter, and in Revit 2023 and up the builtin IfcExport parameters.)
If certain parts need to be another IFC class, model them seperatly (or as shared nested families.)
- Michel