With XACT, you can pitch bend a sound such that it plays slower or faster, and bends in pitch. You can also do this with XAudio2.
Neither XACT or XAudio2 have built-in code to do pitch invariant time stretching. However, if you had access to XAudio2, you could implement some form of time stretching in C# yourself, and feed the stretched audio to XAudio2, but with XACT under XNA, you cannot push your own sound buffers at the mixer, so there's no way of getting there.