I would like to set another namespace default for Special:Newpages. I my
case i would set it to 'all'. Is it possible to do that without changing
mediawiki code?

No, there does not seem to be a server-side option to modify the default in the code, see https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/master/includes/specials/SpecialNewpages.php#301

However, you can preset the dropdown using a query, e.g. https://en.wikipedia.org/wiki/Special:Newpages?namespace=all

HTH,

-- [[ cs:User:Mormegil | Petr Kadlec ]]