Is there a way to create a high LOD area in a specific area?
There’s no way to increase the level of detail in a certain area beyond the highest level included in the tileset. If you’re interested in adding a higher level of detail, you can capture your own photogrammetry using just your phone and free software. There are a lot of tutorials on that out there if you’re interested.
If you’re asking about making an area of a tileset stay at it’s highest level of detail, you may want to adjust the Maximum Screen Space Error parameter on the tileset. This setting relies on distance from the camera and can’t be used in just one area, but may be helpful for your use case.