Absolutely you can do it yourself on the web site..I just did!
When you view your reservation, if you are more than 3 days out from sailing, there is a link to
modify your reservation -- transfers, air, etc. I just added one leg of the transfers because that's all I needed. My res is paid in full, so I did need to pay the $35 right then...printed it out & also received an email confirmation from
DCL within minutes.
Only caveat is if you used a TA - then they may have to do it for you in some cases...
.