When you look at campaign members, you can add leads or contacts as members. I have a need to add what we call "Examiners" as campaign members. Examiners is a custom object we have built in our org to house information about all of the examiners that work for us. We are doing some marketing campaigns to those examiners to potentially increase the amount of work they do (as contractors) and we want to track our efforts there. Is there any way to do that?
Unfortunately you're limited to just leads and contacts/person accounts when dealing with Campaigns.
You have a couple options, first is to get rid of the Examiner object and just use contacts (it sounds like Examiner would be a type of Contact). If there are issues preventing that sort of change you might also consider having a developer build a sync that copies the examiner details into a mirrored contact that could be used for campaign building purposes.
In a more general sense, if you can use a standard object for your purpose, go with it. You'll have a much easier time taking advantage of the CRM features baked into the platform that way.
Adding a quick reference here to this idea on the Trailblazer community: https://success.salesforce.com/ideaView?id=08730000000Lg1CAAS
As it stands: Referencing Campaign Member is not supported for lookups or master/detail relationships.