I believe there is a way to use an adapter.
But.
The iDevices self-identify to the carrier. So, Sprint will know that you are using an iPhone SIM in an iPad. At the minimum, you will get a note from them to stop. At the worst, they'll cancel your phone contract. Of course, this is even if you can get it to work...
Ever since the iPad first came out - there has been no way to use iPhone data SIMs on an iPad. Sorry.
Marilyn