Development of Creative Commons licenses

[Lionel KW Davoust <lionel.davoust AT lionelkw.com>]

Hello all,

I'm a French writer and translator... I came across the CC website while 
looking for a license that would be more permissive than Wizards of the 
Coast's Open Game License, in order to publish and license role playing 
game content... I must say CC's work is impressive!

I would like to ask a question - I'm sorry if it's been debated already, 
I didn't manage to find the subject in the archives - about the CCPLs. 
Are they under copyright by CC? The fact is I'd like to add a clause 
specifying how my original authorship should be expressed, to the CCPL. 
Am I allowed to do this? I know the OGL forbids any modification to the 
license, I was wondering if it was the same with the CCPL.

Thank you very much for your help!

Best,
Lionel