Yes actually,

You just have to write a hook function for the CCK module. The hooks that are used by this module are:

hook_uc_import_csv_row() - Called for every row in the CSV thats is processed

hook_uc_import_csv_fields_list() - Provide an array($data_key => 'Display name') to add options to the select lists on Step 2

hook_uc_import_csv_filename_tokens() - Provide tokens that can be used when building an image file name on Step 2

YOu just need the first 2. hook_uc_import_csv_fields_list() to add your fields (or any CCK fields for the chosen content type) to the drop downs on step 2 and then hook_uc_import_csv_row() to generate the XML as per the documentation that Lyle provided for custom fields. If you were defining your own XML structures completely you'd also have to define a hook for reading in the XML and updating the DB.