You can stop over in Maradi for a short visit or just a good night's rest en route to or from Zinder or Diffa, or use Maradi as a jumping-off point to cross the border into Nigeria on your way to Katsina or Kano. There is supposed to be a Nigerien consulate in Kano; there is no consulate for Nigeria in Maradi, so you'll need to have any necessary documents before you arrive. It may or may not be possible to transit north towards Agadez from Maradi; if it is possible, the method will be neither straightforward nor pleasant i.e. a series of bush taxis on desert tracks, so you're better off trying it in Zinder or, better yet, from Konni, where you can take the bus.
