Can I write I worked as a referee in a journal in my CV? I have recently reviewed an article. I am applying for a Ph.D. position nowadays so I am upgrading my CV and writing the professors. Is it ok if I write that I worked as a referee and mention the journal name in the CV? (As I have a part named 'Scientific and Community services' in my CV.)
Also, this is the same journal where all my articles have been published until now
Yes, you can do that, but it won't mean a tremendous amount in any evaluation. But probably better to list it than not. And the section you suggest is the correct place for it.
It is immaterial whether you have published there or not, assuming, only, that the journal is reputable.