Setting aside the choice of typed/xml vs. xml, I realized I also did not know the answer to the original question.
I found that this works:
#lang typed/racket
(module untyped racket/base
(define five 5)
(provide five))
(require/typed
(prefix-in u: 'untyped)
[u:five Byte])
(add1 u:five) ;-> 6
However, it's not completely clear to me if it is supposed to work: I've opened `require/typed` duplicates effects from expanding `m`; docs unclear · Issue #1378 · racket/typed-racket · GitHub to follow up.