It seems that :db/add supports lookup node-refs in the subject position for existing entities, as the following test passes:

(deftest existing-add-ref
  @(d/transact *conn* {:tx-data [{:id "existing1"}
                                 {:id "existing2"}]})
  @(d/transact *conn* {:tx-data [[:db/add [:id "existing1"] :ref [:id "existing2"]]]})
  (is (= {:id "existing1",
          :ref {:id "existing2"}}
         (d/entity (d/db *conn*) "existing1"))))

(deftest existing-add-ref-to-new
  @(d/transact *conn* {:tx-data [{:id "existing"}]})
  @(d/transact *conn* {:tx-data [{:id "new"}
                                 [:db/add [:id "existing"] :ref [:id "new"]]]})
  ; BROKEN: Resolving the lookup ref does not work, it is stored as-is - likely b/c the entity cannot be looked up
  ;         since we are only just creating it
  (is (= {:id "existing",
          :ref {:id "new"}}
         (d/entity (d/db *conn*) "existing"))))
; => Expected {... :ref {:id "new"}}, Actual {... :ref [:id "new"]}