It is possible to change the migration templates? I have different fields from the template. Also, do I need to create first in Cloud or Customer the field before migration?
As you will read in the first sheet of the migration template Users are expected not to manipulate the format if the file.
If you have a requirement to include new fields in the template you will have to start from the application by creating extension fields in the relevant object.
In case of kut fields they will automatically adapt into the freshly downloaded templates. The migration workbench is designed to do so.
In case of pdi/cloud application studio created extension fields, relevant process extension scenarios have to be implemented for then to incorporate into the migration templates.
Hope this helps