How hard would it be for Chevy to put a VW diesel engine into their Cobalt and come up with a 50 mph car? Not very. Or for Dodge to offer a little diesel in their Dakota, alongside the Jeep CRD. Ford can roll over and die as far as I’m concerned.
I’d like a Jeep Wrangler with a diesel...but I don’t think there is one out there.
Probably not very hard, but "not very costly" might be a different story. Multiple variants of the same vehicle WILL cause costs to go up. Is it worth it to you to pay an extra $3,000 or $5,000 for that vehicle because they offer it in a diesel engine as well? If there is sufficient demand for these alternatives that you would like to see for it to be worthwhile, then Detroit will make them. If there isn't sufficient demand, then having the government mandate these changes will result only in market distortions and higher prices for everybody.