mixing calenders

Well, I think you can include as many wp calendars (the one that comes with the program) as you want. But things like Event Calendar 3 seem to allow only one instance.

No idea what you mean by the second part of the question.

I mean that in our association we have several activities in different domains, visits, meetings..
We would like to keep them seperated in different ‘categories (-calanders)’ but have one which regroups them all ..

The default WP calendar – as far as I know – is showing all your posts, and I don’t know of any way of filtering posts by categories or any other criteria.

Well, the problem with what you want to do is that wp’s not set up to act like Outlook. I don’t think you can do what you want to do with wp. Or at least not with wp alone.

You could use wp for the backend of the site, and use one or more of the available scheduling programs to do the calendaring – they could probably be themed to match. That’s about the only way I think it could work.