How can I make a php file that I'm including have automatically a certain namespace assigned, without me having to add namespace in the file at the top?
This file would be a "template" used to build the site output..
This is simply not possible. PHP's namespaces work per-file.
If this is really what you want, you might be able to use code generation to write multiple files with similar content with a different namespace declaration, but you should review your design and perhaps add more details about what you are trying to achieve.