Per the Marketo Documentation, it is only possible to natively sync SFDC Custom Objects to Marketo that are associated with the SFDC Lead, Contact or Account objects.
However, there is a possible workaround for this. You can create a place-holder Marketo Custom Object, and then sync the data from Salesforce to Marketo via REST API.
Using this SFDC Documentation
This reviews how to set up a workflow in Salesforce to send data to Marketo with REST API.
This is commonly used in the context of SFDC Custom Objects associated with the Opportunity Object in SFDC. Clients create a Marketo Custom Object to send this SFDC Custom Object data to.
This will also give you the option to trigger off updates to the Marketo Custom Object.
Who This Solution Applies To
Clients with SFDC and Marketo Custom Objects