Nissan is not on an exclusive contract. They are however primary on the build allotment, so Nissan gets all they build, if they can sell them. If they can't cummins has to option to sell them to other buyers.
There is one fabricator near me who is planning on a 5.0 Cummins drop-in kit for the E-series, however Cummins hasn't been able to commit to supplying him yet (since Nissan still has time to fill their allotment) and he's finishing up on a Cummins 6.7L turn-key conversion package for the E-series currently. And also doing a build for SEMA.