There may be some aspects of your situation that I do not yet understand. In particular I am not clear whether there is any BGP within the NSSA area.
But it is pretty clear that there is not any way to advertise BGP routes redistributed outside the NSSA into the NSSA. The whole point of NSSA is that externals from other areas are NOT advertised into the NSSA.
I believe that the solution to your situation may be to configure the NSSA so that the ABR advertises a default route into the NSSA. The NSSA ABR does not automatically advertise a default route, but can be configured to advertise a default route.
HTH
Rick
HTH
Rick