Custom fields can be populated in the ‘Post Custom Fields’ settings field, from importing rule settings.
More help on this:
Set the custom fields that will be set for generated posts. The syntax for this field is the following: custom_field_name1 => custom_field_value1, custom_field_name2 => custom_field_value2, … . In custom_field_valueX, you can use shortcodes, same like in post content. Example (without quotes): ‘title_custom_field => %%item_title%%’. You can use the following shortcodes: %%item_content%%, %%item_tags%%, %%pinterest_share_button%%, %%twitter_share_button%%, %%google_plus_share_button%%, %%facebook_share_button%%, %%random_sentence%%, %%random_sentence2%%, %%item_title%%, %%item_url%%, %%item_read_more_button%%, %%item_image_URL%%, %%item_excerpt%%, %%item_show_image%%, %%item_show_all_images%%