Yes, you can do that enablig crossposting (http://b2evolution.net/man/crossposting). It's only impossible to move posts into Meta categories, but you can assign both main and extra categories from any other collection.

Sorry, I just forgot to attach a screenshot to my previous comment showing how the categories fieldset is presented when crossposting is enabled.

Thanks

Yes - that help do the 'trick'.

I was looking for this option but forgot to check in the overall blog settings.

Kind regards.