XP-2 does cross-process in E-6, though it yields slides of low-ish contrast and of an interesting greenish-black tone. You'll also probably be charged a bit extra for cross processing.
What I've done that works (with some experimenting) is the following:
Expose the film at either 200 or 400 (try both on the same roll).
Process the film in your Black and White developer of choice, treating it as Tri-X being pushed to EI 1600. (Whatever your specific developer reccomends as that time).
Then, WITHOUT fixing, wash the film as completely as possible (30-40mins in running water). You can then open the tank in room light, and expose the length of the film to the light for a minute or two. Dry the film in subdued light.
Roll it back into the canister once dry, and take it to wherever you do your normal C-41 processing.
The result should be a black and white positive, with a fairly good image of slightly high contrast. It'll take some experimenting, but it certianly works.