CAPT OF AN ACR LGT DECLARED AN EMER WITH ATC AND DIVERTED TO LAND AT AN ARPT NOT ON THE COMPANY'S APPROVAL FOR THAT TYPE OF ACFT.

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FLT DEPARTED SFO ENRTE TO ORD. PLANNED ARR FUEL WAS 8000 LBS. WX FORECASTED TO BE GOOD; CHANCE OF A TSTM AFTER OUR SCHEDULED ARR. NO ALTERNATE REQUIRED. FLT TIME WAS A LITTLE LONGER THAN NORMAL DUE TO REDUCED WIND. WHILE ENRTE; RECEIVED 2 REROUTES FROM ATC DUE TO DETERIORATING WX AT ORD. MESSAGES TO DISPATCH WERE SENT AND WE RECEIVED MKE AND RFD AS ALTERNATES. THESE 2 REROUTES WHICH TOOK US TO THE N COST 2300 LBS OF FUEL. COMPUTER NOW SAYING ARR AT ORD WITH 5700 LBS. A TSTM WAS NOW OVERHEAD ORD. WE RECEIVED CLRNC TO GO DIRECT TO AXN VOR AND HOLD. WE WERE STILL 450 NM FROM ORD. AFTER SEVERAL INQUIRIES FROM ATC; THE WORD WAS INDEFINITE DELAY. DISPATCH ADVISED. WE DECIDED TO DIVERT TO MSP. ENRTE TO MSP ON THE GOPHER ARR; RECEIVED HOLDING INSTRUCTIONS AT BRN VOR. MSP APCH NOT ACCEPTING ANY ACFT DUE TO TSTM OVERHEAD MSP. DISPATCH ADVISED. AFTER LOOKING AT SEVERAL ARPTS INCLUDING MSN; DLH; AND GRB; DECIDED TO HEAD BACK W TO FSD. THERE WAS NO WX BTWN US AND FSD; I WAS FAMILIAR WITH THE FIELD; COPLT HAD APCH PLATES; AND I KNEW WE COULD GET IN WITH NO HASSLE. FSD WAS NOT IN THE ACFT'S COMPUTER DATABASE BUT WE BUILT THE FIELD AND RWY MANUALLY. ABOUT 20 MINS OUT; DISPATCH INFORMED ME THAT FSD WAS NOT AN APPROVED ALTERNATE FOR OUR ACFT AND ASKED US TO LOOK AT SUX. ALTHOUGH ONLY SLIGHTLY FARTHER; WE HAD NO APCH PLATES AND DID NOT HAVE ADEQUATE TIME TO REPROGRAM FLT COMPUTERS (SUX WAS NOT IN FLT COMPUTER DATABASE EITHER). LNDG AT FSD UNEVENTFUL ON RWY 21 (9000 FT). WIND 190 DEGS/20 KTS. WX WAS AROUND 4000 FT OVCST; GOOD VISIBILITY. SINCE FSD WAS AN UNAPPROVED ALTERNATE FOR THE ACFT; I DECLARED AN EMER WITH ATC AND USED MY EMER AUTH TO LAND. BLOCK IN FUEL AROUND 5300 LBS; WHICH WAS WELL WITHIN MY COMFORT MARGIN.