Hey Eric--for some reason I was under the impression that you
could re-map fields in Marketo, but I tried to test it just now and I couldn't figure out how, either. I think the best thing to do is to either open a support ticket, or hide the field you created in Marketo, create a new field in SFDC, and then let it automatically sync down into Marketo.
Sorry, I know that's probably not the news you were hoping for
😞