We don't have any supported method to add modules to Apache. What are you trying to install?
If you really need a custom Apache setup, you can switch your VPS or dedicated server to "no web server" (in the Configure Server section of the panel) and install Debian's apache2 package. Note, however, that this will disable all DreamHost management of the web server — in particular, the Manage Domains section of the panel will no longer work for the VPS / dedicated in question. We don't recommend this course of action unless you're really happier doing everything yourself.