This is currently not possible. There are two issues filed related to this:
I think we will support it in the future, but I can’t say it’s been a big priority. In the meantime, the first issue linked above describes a small code adjustment you can make to get the search to appear on the homepage.
You should be able to access a site-wide search by adding /search to the end of your journal index page url, as explained in this forum post. I’m not sure if that’s exactly what you’re looking for.
Best,
Amanda Stevens
Public Knowledge Project Team