you could modify the ext cab frame to accept the crew body, they both have the same wheelbase. if you want to change the bed then youd need to stretch it after the cab.
my buddy had a crew cab with all the interior he was gonna build but decided not to he was tryin to sell, i'm pretty sure he's still got it if you're interested in it PM me and i'll get ya the info
I seem to recall that a the 4 door was available to the Mexican market. Don't know what's involved in getting one (or parts). I know they also had a dually available in Mexico. Don't remember if that was first or second gen. S-10 though.