Don't import/export, it doesn't do what you think it does. Export exports all of your data. Import imports data from other licensing systems (not LimeLM) and generates new LimeLM keys with the data.
When you add a custom license field all existing keys will have the *possibility* of having that field set. The default value has no effect on existing keys. It only effects newly generated keys.
To edit existing keys, edit them in the interface or using the LimeLM web API. It's your choice.