You can just type in whatever you want, even if it's not in the pick list, and it will change the data value.
Once that value is populated for at least one lead (i.e. after you type it in, run the campaign once and your new value will be available in the pick list)
The drop-down is more suggestions of field values than a strict picklist (except in situations like asset names, sales people from CRM, etc). It tries to suggest unique values it currently sees existing on people in the database. If you have records come into Marketo with the new values saved in "Person Source" it should suggest them then after a while. It's not something that is synced with picklists in Salesforce.