ImportWP Pro gives you the ability to import from both XML and CSV files into custom fields from basic text to wordpress attachments when importing WordPress posts, pages, custom post types, and users.
Click on the add button on the right hand side of the custom field meta box (+) to add a new custom field record, within this record select the custom field you want to add, for example if you are using Yoast SEO plugin you can choose to populate fields such as focus keyword, seo title, and seo meta description.
If you cant find the custom field you are looking for, you can check the Enable text field checkbox that will allow you to manually type the custom field key you want to import to.
Once you have set or selected the custom field, you can now set the value by clicking on the select button under the Value column to load up a modal allowing you to select which data from either the csv or xml file that you want to populate with.
You can choose to import the custom field value as it comes from the xml or csv file, or if you are wanting to import an attachment (for example if this is a url to a file), you can change the field type dropdown from Text to Attachment. If choosing attachment you then want to select what value is stored in the custom field, this is set via the Return Value dropdown which can either be the attachment ID, or attachment URL. The attachment can be fetched from many different methods such as importing an attachment via FTP, or importing the attachment from a Remote URL or local file on the servers file system.
An example below of importing a custom field named custom_image which is downloading an attachment from a remote url, inserting it into the media gallery and storing the attachment url in the custom field: