the standard solution is to use Data Protection.
With this solution, data you want to protect is extracted before new data is loaded, and then appended to the DAT file. In this way all data is loaded by "protecting" your existing data.
You need to check in which intersection users are entering data so you can protect it. Customers usually customize it but this depends on your reqs.
You can also see how it works if you take a look to the Adapter Action Script "LOAD"
You can find how to setup in the adapter readme guide.
Just to add a note to the above answer, when FDQM checks for the protected value it is not dimension specific, so you need to ensure that the string you are protecting is not used in any other dimension otherwise you could be saving more than you expected.
Thanks a lot for this answer