is it possible to use the personal license bought from steam without opening it via steam? I mean, bought the personal version from steam and downloads the C2 from scirra site?
The license should be valid regardless of where you purchased C2. Look at the license and see if it says anything about STEAM.
If it doesn't and you don't want steam then download C2 from Scirra website and put your license in that folder.
I keep a backup copy of the license just in case.
I got my license a long time ago so I don't know if they have changed anything since then.