[ROS2][Nav2] Is there a way to expand the global costmap ?

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I am working in ros2 foxy in ubuntu 20.04 with a custom made robot.

In my current setup i use an imu and odmotry (produced by diff_drive) and a localisaton system outside of the robot (like gps). My goal is to map an environment using only the footprint of the robot ( as i don't have any other source of information ). To do that, the only way i found was to publish a large empty static map using a map server as global costmap and have the robot move around. This leaves a tray of "free cell" in my global costmap. I then go around the area i want to map and then save it and fill the inside with a python script. This leaves me with a map containing "free cells" and "unnown cells".

The only issue I have with that is that I don't know how to expand my global costmap : If my robot goes to far, it's radius won't be "printed" on any map and won't be saved.

Is there a way to expand the global costmap in a more dynamic way ?

my nav2 global costmap config :

global_costmap:
  global_costmap:
    ros__parameters:
      publish_frequency: 1.0
      update_frequency: 5.0
      global_frame: map
      robot_base_frame: base_link
      use_sim_time: False
      robot_radius: 0.5
      resolution: 0.05
      track_unknown_space: true
      plugins: ["static_layer", "obstacle_layer", "inflation_layer"]
      obstacle_layer:
        plugin: "nav2_costmap_2d::ObstacleLayer"
        enabled: True
        observation_sources: scan
        scan:
          topic: /scan
          max_obstacle_height: 2.0
          clearing: True
          marking: True
          data_type: "LaserScan"
      static_layer:
        plugin: "nav2_costmap_2d::StaticLayer"
        map_subscribe_transient_local: True
      inflation_layer:
        plugin: "nav2_costmap_2d::InflationLayer"
        cost_scaling_factor: 3.0
        inflation_radius: 0.55
      always_send_full_costmap: True
  global_costmap_client:
    ros__parameters:
      use_sim_time: False
  global_costmap_rclcpp_node:
    ros__parameters:
      use_sim_time: False

I use an obstacle layer in the config file but I don't have anything publishing on the /scan topic, I just use this layer to "print" the footprint of my robot on the map.