on stackoverflow I aske a question. There are many good answers but I can not accept just one of it. I would prefer to create my own answer where I summarize all answers. Is that allowed?
Yes, that's fine. The idea is to get the best possible answer to the question to the top of the page so future Web searchers will find it quickly. If you need to combine several answers into one to do that, then feel free to do so. Just make sure you give attribution if you're copy/pasting someone else's answer (just a link to their answer).
Also remember that normal voting rules still apply, so if other people see you doing this and don't think it provides value, they're free to downvote it. Try not to take offense to this, just use it as feedback and adjust the way you're answering.
A good example is in Hidden Features of C#?, although the summary should be in an answer instead of the question.
Another approach is to add a comment to the "rightest" post, suggesting the author update to include the other info, with attribution where necessary.
Then you can up vote all contributors to the final answer, and accept the now complete one
That will potentially minimize posters potential frustration, and better align the resulting votes with the primary source of the answer. And you'll even get 2 points for accepting, since it won't be your own response.