About license agreement

Hello,
I have a question referring to the license agreement of WP.
I found one WP plugin, it is free, but the author also sells additional modules for this plugin at his site.
From the readme.txt file:
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

So the question: may I take this original plugin, develop and integrate most of these additional modules myself for free, but some own modules I’d like to distribute for money?
As I feel it doesn’t violate GPLv2, am I wrong?

Thanks.