In the routing system you can't do this. What you can do is path aliasing.

Inbound: `/{bar}/foo/{baz}` -> `/foo/{bar}/{baz}`

Outbound: `/foo/{bar}/{baz}` -> `/{bar}/foo/{baz}`

You need a pattern to apply a regular expression on the path inbound, so `/{bar}/{baz}` is only possible if the parameters themselves have a distinctive pattern.

I didn't find a tutorial how to build a pathprocessor, but you can use this example from core for the scaffolding:
http://cgit.drupalcode.org/drupal/tree/core/lib/Drupal/Core/PathProcessor/PathProcessorFront.php

and register both as tagged service in *.services.yml

    path_processor_front:
        class: Drupal\Core\PathProcessor\PathProcessorFront
        tags:
          - { name: path_processor_inbound, priority: 200 }
          - { name: path_processor_outbound, priority: 200 }
        arguments: ['@config.factory']