I believe
DCL transfers MCO -> Port Canaveral can be added at MCO on departure morning. I don't recommend waiting that long, but I believe it's possible.
I have also read of passengers who added post-cruise transfers (PC -> MCO or PC -> WDW) during the cruise, prior to debarkation day. In either situation, transfer charges are added to your onboard account.
As mentioned, the only time you can add DCL transfers online is at original booking. You or your TA needs to call to add them at any other point.
Enjoy your cruise!