How to override Add to Campaign button in Salesforce?

1. Create a VF page with standardController="CampaignMember".

Sample Code:

<apex:page standardController="CampaignMember" extensions="Sample">
</apex:page>

2. Go to Setup --> Build --> Customize --> Campaigns --> Buttons, Links, and Actions.

3. Click Edit.

4. Select the VF page created.

Cheers!!!