It's a reasonable request, but not enough people pin airfields to their route for us to be able to justify adding a top-level item in What's Here for the action. Therefore it would be a second-level button, if anything:
1. Right-click airfield
2. Click Routing
3. Click Pin to Route
As for adding all airfields within a certain corridor, there are so many minor airfields out there that I promise this would not work very well. You'd end up with dozens, if not hundreds for a long route, and everybody's idea of what is a useful airfield to them is different.