These files should be created by an electrical schematics application such as Mentor Graphics or Cimteam. You do not have to have the files to model a wire harness. You can manually connect wire paths to connectors.
As stated in a reply those files typically are exported from a 3rd part ECAD system and imported to Solid Edge using the wizard. If you know the format you can use any text editor to create your own. The format is defined under the Solid Edge\Program\Preferences subfolder using the file SEHarness.txt file. That tells you the formats for each ECAD system and is editable as well.