How can we install civicrm extension which is not available for drupal
You need to first get into the extension directory, and download the extension directly. Then you can install the extension from Manage Extension Page.

wget https://github.com/civicrm/civihr/archive/1.3.0.zip
unzip 1.3.0.zip
mv civihr-1.3.0 civihr


You can now move to Administer&gt;System Settings&gt;Manage Extensions Install CiviHR and you are good to go.

This link can help you further.