@jacquesdpz maybe:

(require '[malli.core :as m])
(require '[malli.registry :as mr])

(mr/set-default-registry!
 (mr/composite-registry
  (m/default-schemas)
  {::user-id :int
   ::user [:map
           ::user-id
           [:name :string]
           [:age :int]]}))

(require '[malli.experimental :as mx])
(require '[malli.generator :as mg])

(mx/defn get-user :- ::user [id :- ::user-id]
  (assoc (mg/generate ::user) ::user-id id))

(get-user 123)
; => {:kikka/user-id 123, :name "8p28ySz", :age 2405534}