You will have to refer to the conditions of the tickets and the retailer you bought them from for returns/refunds.

There is no mechanism for transferring them to Eurail pass reservations.

Don’t just assume that the advertised Eurail pass rate is all you need, be aware that paid reservations apply to Eurail in many countries, for TGV there are 2 rates €10 or €20 per trip, €10 has a quota and disappears when they are sold with €20 then available.

International high speed trains from France have even more expensive reservations and small quotas that once gone make those trains unbookable for passholder rates.

EUrail will only pay off-if at all-if you use it for at least 4 days and over EURope-for just a normal inland return most likely not. This even without the potential other hickups-esp in FR and even more in ESapana-detailed above.