You Do Need power plug adapter while traveling from Niger to Namibia.

Here is Why You Need An Adapter

The outlet and plug type used in Namibia are D and M and in Niger are C, D, E and F. This is how they look like.

Niger

In Niger, outlets of type C, D, E and F are in use.

Type C

Type D

Type E

Type F

Namibia

In Namibia, outlets of type D and M are in use.

Type D

Type M