Hi Alexia,
You may be able to use the Person Notes field for this, it sync to the Description field on the lead and contact object in SFDC. Please check with your team carefully first bcause they might use this for a diffierent business purpose to the info which you wish to add and you could potentially overwrite valuable data if you are uploading existing leads.
Alternativley you can create a new text field in SFDC on both the lead and the contact objects and name them the exact same. Naming them the same means that they will only be one field in Marketo that sync to the lead and contact object in SFDC. When creating them make sure that the profile associated to the Marketo sync user has access to read and write to this field.
Wait 5 minutes until your next sync and you should see this field created in Marketo, then use this field when uploading your list to store the event notes.
Cathal