How do I create a mileage upload file where mileage has already been claimed in Fusion?

Summary:

I wonder if someone is able to help me please?

I am trying to upload mileage into Fusion Expenses for employees that have already claimed mileage expenses in Fusion.

Content (please ensure you mask any confidential information):

This data to be loaded was from another system but needs to be loaded to comply with HMRC rules on mileage rates. I am able to create a file with person number and number of miles and this uploads successfully but I need to tick the option to allow update of existing records as the upload seems to overwrite the miles claimed rather than add to what has been claimed. However, when this is uploaded, on the employee view of cumulative mileage, it removes the value (pounds) of mileage claimed and rests to zero. It appears that it may be