I'll try now... If there are further questions, I'll come back :)
There I am, I was on the wrong way...
Adding new attributes to an existing eZContentClass is possible with the solution you explained before.
But I need to add new attributes programmatically to eZContentObject.
I'd like to try with
$new_attribute = eZContentObjectAttribute::create($contentclassAttributeID, $contentobjectID); but where do I get "$contentclassAttributeID" from?
You'll need to create class attributes first. You can't store objects attributes without defining these attributes as class attributes, at least with the default API.
But you can always create subnodes that could work as pseudo object attributes.
In backend creating new article of class 'cars_article' I have now a new field 'keywords'. BUT if I want to add keywords to an existing article, I don't have the field. What's wrong here?
How can I add keywords into the article programmatically?