Custom Objects created in your Salesforce instance can be part of Marketo too. Here's how to set it up.
Admin Permissions Required
Enable Custom Object
1. Click Admin and the Salesforce Objects Sync link.
2. If this is your first Custom Object, click Sync Schema.
3. Click Disable Global Sync.
An initial sync of the Salesforce custom object schema may take a few minutes.
4. Drag the custom object you wish to sync into the canvas.
5. Click Enable Sync.
6. Click Enable Sync again.
Don't forget to re-enable your global sync!
7. Go back to the Salesforce tab.
8. Click Enable Sync.
9. To view all of your Salesforce custom objects, click Admin and the Salesforce Objects Sync link (same as step 1 above).
Marketo only supports custom entities that are linked to standard entities one or two levels deep.