Colleen,
I'm a bit confused as to which fields we're talking about.
If you only want to use Lead Source, that's already synced to Marketo/SFDC.
You can field block Lead Source from being overwritten by subsequent forms with the Admin > Field Management > Field Blocking tool. There are tons of instructions on this already. Just drag in LEad Source to the center of the screen, select Field Blocking from the menu, then select the Import Sources you want to block. Usually you block Import List, Form Fill Out, etc.
This should ensure brand new leads are assigned a Lead Source while future form fill outs do not overwrite this.