Adapters are expensive, but can be found.

What you need is 1/2" pedals, common for BMX and other one piece cranks. I know of no clipless pedals for 1/2"

EDIT:

A pair of adapters are available here for $45 or so...

http://sheldonbrown.com/harris/pedals.html

Another EDIT:

There are also adapters available to replace the one piece crank BB with a more traditional 3 piece crank.