You can create custom fields only if the target record type for this field is customizable and if you have the appropriate privileges in Microsoft Dynamics CRM. More information: Customize and publish during import in Permissions required for customization tasks.
If a field already exists in Microsoft Dynamics CRM with the name that you entered, Microsoft Dynamics CRM prompts you to type a new name.
If a record is related to another record, that record has a field that points to a field in the related record. The field that points to another field is called a lookup field.
By default, Microsoft Dynamics CRM temporarily adds each value from the set of values in the column of the source file to the CRM Option Values list, and maps these values automatically. If you do not want to include a particular value from the source file in Microsoft Dynamics CRM, click Ignore in the CRM Option Values list next to that value from the source file.
For example, a "Rating" column in a source file contains three values: "Hot," "Warm," "Cold." You want to create a custom field in Microsoft Dynamics CRM for the Rating column including only "Hot" and "Warm" while excluding "Cold." do this, in the CRM Option Values list, for "Cold," click Ignore. The completed import will only include the values "Hot" and "Warm" for the Rating field in Microsoft Dynamics CRM.
Note
When you create custom fields for any existing or for new custom record type, Microsoft Dynamics CRM creates and publishes the custom fields and adds the imported data to these fields. However, Microsoft Dynamics CRM does not automatically add these fields to the record type form. To see the data imported in these fields on the record type form, navigate to Customizations after completing the import, and then add the fields created during the import to their respective record type forms.