Migrate user passwords from drupal 6 to already existing drupal 8 site
If you only want to migrate users and their passwords then you can do the same by using drush commands.
Firstly you have to install certain modules:
1.Migrate Upgrade
2.Migrate Plus
3.Migrate Tools

Then you have to create a manifest.yml file at the root of your d8 site.
The manifest.yml must look like this:

# user 
- d6_user 
- d6_user_profile_field 
- d6_user_profile_field_instance 
- d6_user_profile_entity_display 
- d6_user_profile_entity_form_display 
- d6_profile_values:user 
- d6_filter_format 
- d6_user_role 
- d6_user_picture_entity_display 
- d6_user_picture_entity_form_display 
- d6_user_picture_file 
- d6_user_picture_field 
- d6_user_picture_field_instance 


Then run the following drush command:

drush migrate-manifest --legacy-db url=mysql://d6user:d6pass@localhost/drupal_6 manifest.yml 


For further information follow the following page:https://www.drupal.org/docs/8/upgrade/upgrade-using-drush