Hello Vio,
In principle, providing that you are using one of the latest PhotoScan versions, you can do the following:
- right-click on the dense cloud label in the Workspace pane and uncheck "Set as default" option,
- resize bounding box in order to cover the missing area (note that you need considerable number of tie points in the box in order to properly complete the reconstruction),
- run Build Dense Cloud operation,
- select both lines related to the Dense Cloud in the Workspace pane, right-click on selection and use Merge option.