It won’t be possible out of the box.
Customization will be needed either on the frappe tree.js and employee query etc OR you can do a little javascript tweak in the employee_tree.js.
Here’s a sample tweak
onrender(node) {
let page = $(document);
if(!node.is_root) {
frappe.db.get_value("Employee", node.data.value, "department")
.then((r) => {
if(r.message.department){
let x = page.find(`span[data-label="${node.data.value}"] .tree-label span.text-muted`);
x.text(` ${ x.text() } ( ${r.message.department} )`);
}
})
}
}