Migrating a site with a module that doesn't have a version for latest core version
You simply cannot. If the module has a Drupal 8 version you can use it just after enabling the module. But if it doesn't contain a Drupal 8 version you cannot enable or use with your Drupal 8 site installation. Only thing you can do is find an alternative module for the unsupported module that you have used. Or you can change the unsupported modules codes with supported Drupal 8 syntaxes. But I never encourage to do so. Because 


It effect authors copyrights.
You wouldn't be able to update anymore or if you update your module forcefully all the changes you have done will revert back to the original state.


So think twice before doing a change like that. Hope this helps for your question.