You cannot assign an owner without it syncing to SFDC. You could potentially create a field in Marketo only that will stamp it that way, then use it later to assign the lead in SFDC.
If you don't want to sync the lead, then you can just not sync it over. You can also mark all incoming leads as "Do Not Sync=TRUE" until you are ready to sync it.
Is there some reason you need the Lead Owner on there?
Yeah, I want to send out prospecting emails for my sales team and I don't want to have to clone each email and send it off as a different AE. What are other ways around this without having to clone each email?