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

Here is Why You Need An Adapter

The outlet and plug type used in Namibia are D and M and in Pitcairn Islands is I. This is how they look like.

Pitcairn Islands

In Pitcairn Islands, outlets of type I are in use.

Type I

Namibia

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

Type D

Type M