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

Here is Why You Need An Adapter

The outlet and plug type used in Sierra Leone are D and G and in Namibia are D and M. This is how they look like.

Namibia

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

Type D

Type M

Sierra Leone

In Sierra Leone, outlets of type D and G are in use.

Type D

Type G