Can I pass my collection agency's charges on to the patients the collection agency is pursuing?

Yes, you can charge the patient the collection agency recovery fee. Make sure, though, that you disclose the fee to the patient in advance, perhaps in a financial policy description you distribute to all new patients and in the statement they receive. This will alleviate the payments that the practice is making for these collection efforts. This might even serve as an incentive to get patients to pay sooner rather than later. Let them know that if they do not pay by a certain date, you will be forced to send the account to a collection agency - and charge them for the service.