Let's not get ahead of ourselves. Campaign member fields aren't supported natively at all by Marketo, as discussed at length in Greg's Idea here: Have extended Marketo Program member fields and use them to full power (with or without SFDC campaign member field sync)
So the "simplest" way is never going to be simple.
The options are:
- don't do it, and accept that Marketo isn't made to do it (at least yet)
- use a webhook-compatible service (possibly an Apex REST service built directly in SFDC, though it doesn't have to be as it can route through a webhook gateway) that can update SFDC based on data carried in a special Marketo field (Textarea) designed for this purpose
- sync the same Textarea field described above to SFDC (using the native Marketo-SFDC sync) and use Apex to update the campaign member
You are entering Salesforce area here but..
Try creating a hidden text field on the LEAD object in Salesforce and use a Formula field on the CAMPAIGN MEMBER object to copy the value from the lead to the campaign member as, as Sanford says, Marketo doesn't write to the campaign member object atm.