mikerotec wrote: We'd like to start using DKIM authentication on all of our emails.

Is there an interface somewhere for adding the DKIM header to all emails sent by CBsubs?

It's not CB/CBSubs that adds DKIM signature headers, but your mail server. You can see it here e.g. with this Kunena notification email by looking at raw headers.

So you need to configure your email server for DKIM. This is outside the scope of these forums, but there are many good tutorials for that, and some hosting panels support that out of the box (we use Virtualmin GPL here and it does it just fine.