Welcome back, Jessica!
It is so great to see you again here at planDisney!
Jessica, I have been in this exact same situation. The answer to your question is actually .... it depends. In the past, I have purchased
theme park tickets through my husband's place of employment and not directly through Walt Disney World Resort.
When I purchased these tickets I was able to add the
Park Hopper option, as well as an extra day, but I could not do so until we arrived at our resort. Jessica, note that in order to upgrade your tickets, you must have an additional entitlement remaining, meaning that you must add the Park Hopper option before you enter the park on the last day of your visit.
Now, Jessica, when you purchase through a third party you do run the risk of purchasing a "special issuance" ticket. These tickets are unable to be modified or upgraded. I recommend that you reach out to the outlet that you plan on purchasing from to verify your ability to upgrade them at a later date.
I hope that this answered your question!
Wishing you a magical vacation!
Sarah B.