There are many ways in which to achieve this with Eloqua - The common one is when a lead is rejected from sales, you have sales pick a followup action, which can include assigning to Eloqua nurturing. This can be achieved by creating a field mapped between CRM and Eloqua that allows a sales rep to nominate an Eloqua nurture track. On the Eloqua end you can use this field to feed a contact into a specific track.
Also, degraded scores can easily be passed to CRM. You have control over when to send your leads through to CRM from Eloqua so you should be able to feed contacts through to CRM after they have been scored.