I don't believe there is a way Marketo can update the information on the Campaign Member object in Salesforce, (except for the Member Status). The same question was asked about a year ago, but no definite answer to this.
No, you cannot. I doubt this will ever happen. You can speak with your SFDC Admin about your needs and how to populate the values via the API.
A
Anonymous
June 11, 2015
Makes no sense why they would never allow for this since they already can write to the member status field on the object. Sadly, Eloqua has been able to write to this object for many, many years. It has always frustrated me that they refuse to build this in. See the idea here: