go go go

CL-USER> (define-symbol-macro %namespace nil)
%NAMESPACE
CL-USER> (defmacro with-namespace (name &body body &environment env)
           `(symbol-macrolet ((%namespace (,name ,@(macroexpand '%namespace env))))
              ,@body))
WITH-NAMESPACE
CL-USER> (defmacro defun-in-namespace (name lambda-list &body body &environment env)
           (let ((actual-name (intern (apply #'concatenate 'string (mapcar #'symbol-name (append (macroexpand '%namespace env) (list name)))))))
             `(defun ,actual-name ,lambda-list ,@body)))
WARNING: redefining COMMON-LISP-USER::DEFUN-IN-NAMESPACE in DEFMACRO
DEFUN-IN-NAMESPACE
CL-USER> (with-namespace foo (with-namespace bar (defun-in-namespace baz ())))
BARFOOBAZ