You can make them fit. Removing the sportster hubs and lacing softail hubs may be required. Not sure if replacing spokes would be required. No way will the sportster hubs fit on a softail. My wifes bike has a touring laced wheel in back flipped and a softail deluxe wheel up front with the matching FL front end.
Lots of variables,disc,pulleys,offsets to consider. Bearings and spacers would be the easy things after the others. Unless you have the bucks to burn I'd ride the Sportster as is till the softail urge kicks in and then spend the money.