I found the best workaround for my issue.
I wrote the body of the feeditem into a custom field on the parentobject of the feeditem.Then I activated the feedtracking for this field and the feedtracking will create a feedItem automatically with the "sites user". In the standard chatter view you can see that it is an automated feedtracking feed but with a custom visualforce page it is possible to design it the way I want!!
For me this result is ok with and very easy to achieve.