Dreamhost gives you a pre-installed choice of 2 webmail front end apps. Squirrelmail and roundcube. AFAIK neither of them, as installed, let you add additional plugins.
You can however roll your own. That is you can install any webmail app of your choosing with whatever options or plugins you need or want. The downside is that now it's up to you to do the heavy lifting when it comes to upgrades, security concerns and the like.
If you opt to take that route and want the default DNS entry for 'webmail.' to be deleted you have to contact support, the panel will claim that DNS entry is not editable.