Depending on when/how you're getting those dates from FedEx, your best bet would probably be to add that text where the main realtime shipping call is done and the list of shipping methods is built up. SOPC uses the shipping logic in AspDotNetStorefrontCore (RTShipping.cs, FedEx.cs, etc) so as long as you have source code that shouldn't be hard to do.
Otherwise you're going to have to parse through ShippingMethods.Items after it's bound and add your text to the values in there.