Hi,
that is a little tricky and I am not sure if it is possible at all.
The menu entries for all Links are stored in a table called: wcf1_header_menu_item
This table is cashed in the file: /wcf/cache/cache.headerMenu-38.php
You can change the entry for: de.bessieundmarkus.gallery2.header.menu
You can change the links in the table to whatever you want to and than you have to delete the cache file so the data are read newly from the DB Table.
Now the Problem starts. There is no flag for where to open this links.
Entering a JavaScript command fails, I tried it and it did not work.
Maybe there is a plugin with an event listener offering the possibility to change manue links, out of the box I do not have an idea how to set this up easily. Did you ask a question like this to the WCom oder Woltlab support Board?