how to update the costmap manually in Rviz?

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Hello,

I'm wondering if there's a way to manually change the costmap in Rviz.

My goal is: if we have an initial location and a target location, the robot moves as if there's an virtual corridor around the shortest distance path, and everything else, even free space, is considered as obstacles. And this way, if there's a real obstacle on the shortest distance path, the local planner would just stop, instead of going around it.

My thought is to maybe just manually update the costmap, pixel by pixel, so that we can have a costmap representing the virtual corridor as described above, instead of the real costmap. So how is the costmap saved and loaded? Is there a way for us to update it manually? Other suggestions on how to approach this is also welcomed =)

Thanks!