How to add plug-ins on on-premise base?

From what I know, you have to go to Admin console and select plug-ins tab. Then choose the plug-ins and deploy it.

But, to add plug-ins that are not included in the Appian App Market, you have to contact with the Appian Support team.

Do I have the right idea?

What I want to do is add plug-ins on on-premise base.

How do I add plug-ins?  Could I get a specific description of how?

Thank you.