I have created a custom field in Marketo called "Notes Update" that I use to append new notes with the existing data in the "Notes" field in SFDC.
For example, when I want to import a new note to the "Notes" field in SFDC while still preserving the historical data, I import the new note into the "Notes Update" field in Marketo. That import triggers an operational campaign that I've set up that inserts this value into the "Notes" field, which then syncs over to SFDC: