I suppose your question is more related that a user should be able to change values (default value) of an existing migration job without modifying the migration job itself?
If yes then this might be the solution you are looking for:
Example of a MIGRATE_SOURCE_DATA migration job:
Source mapping:
The column “change default” is set by default to “Not Allowed”.
If you change this value for a column to “Allowed” then a user can change the value of this column before executing the migration job:
Execute job:
In the tab “Defaults” you can now see the column where the value for “Change defaults” was set to “Allowed” in the source mapping:
Here the user can change these values and execute the migration job afterwards with these values: