A few things of note. When you add an extended repo, it gives you the ability to install packages from said repo. There is no Fixlet content provided by BigFix for extended repositories out of the box. If you do require Fixlet content, you can engage the services team for this.
There are two separate tasks that you can use to install packages from extended repos:
Task 18 ‘Install packages by using yum’: When you take action on this task, you are prompted to enter the name/s of the package/s you want to install. If left blank, it will update all packages on the endpoint targeted. This task will utilize the locally configured repos on the endpoint, and not the ones configured with the DownloadPlugin.
Task 401 ‘Yum command with CentOS download plugin - CentOS 7 -x86_64’: When you take action on this task, it will also prompt you for input. In this case you can pass yum commands (install, update, etc). As an example, if you wanted to install bind-utils, you would enter: update bind-utils
This task will utilize the CentOS Download Plugin and the repos you have configured through BigFix.
Hopefully this answers your questions.