Are you trying to keep leads in Marketo until they have passed a certain score, before syncing to Marketo?
If so, I believe if you add a lead to a campaign without using the Sync to SFDC function, it will bring the lead over into SFDC, but it will not tie a lead owner to it. Depending on how SFDC is setup this could prevent the reps from seeing the lead. Might be worth testing, if this is what you are trying to achieve.