Hi Karsten,
As far as I know, the user can only add Website page to the portal site Navigation bar.
As a workaround, you can create a new General page in the Website, and then add a Hyperlink in this new page to redirect user to the blog site, here are the steps:
1. Go to the Website page, and click Pages – New Page.
2. Select the General page template, and then fill in the page information.
3. Select the new page, and then click Insert – Hyperlink. Add a Hyperlink to the blog site.
Thanks,
Eben Liu