Am I able to add plugins after my site is completed?

Yes!

If during the initial creation of your site you elected not to use any of the plugins, login to the Optuno Control Panel and make a request to your project manager. Each Optuno plan comes with a set number of plugins included. Just select the plugins you want and Optuno will install and integrate them for you within a few days.

If you've used up all of the plugins available with your package, you can purchase additional plugins for $99 per plugin (one-time fee).