I've solved it. So here is what I did :
Include bootstrap gem in my Gemfile
gem 'bootstrap', '~> 4.0.0.alpha6'
Restart the server and then import bootstrap to /app/assets/active_admin.scss
@import "bootstrap";
Now you can add progress bar or other components to any page of ActiveAdmin. For example for dashboard page, add the following code to the /app/admin/dashboard.rb
render html: "<div class='progress'><div class='progress-bar' role='progressbar' style='width: 25%;' aria-valuenow='25' aria-valuemax='100' aria-valuemin='0'>25%</div></div>".html_safe