You can of course convert Amex membership rewards into Continental or Delta (or Aeromexico for that matter) miles which can be redeemed (well, as much as any miles can actually be redeemed now days) on Northwest.

It's probably best to keep using the Amex card and transfer miles into specific airlines when you want to redeem that way you have a lot of flexibility if you want to fly somewhere Northwest and it's partners fly.

If you want to earn Northwest Worldperks directly then there is only one supplier, US Bank, with its range of cards (all major US airline/hotel Credit Cards are listed on my site http://www.andrewcram.com/frequentflyerCC.html) .