Uber is in the process of making the addition of up to two stops available in the app, but I don't know if the Orlando market has it yet.
The intent of that change is to allow for multiple stops to pick up or drop off passengers on the same ride. For example, a couple of weeks ago, I picked up a mom and two kids, then went to a second location and picked up another mom and kid, and drove them all to a friend's house for a pool day. At both pickups, both groups were standing on the curb when I arrived, so the pickups were fine.
However, shopping trips are not so fine, and most drivers will say no to a shopping request. Shopping takes time, and waiting for shoppers means missing additional rides, which costs the driver money.
Ask the driver if it's okay, agree on a tip amount, and set (and adhere to) a short time limit, and the driver may agree.
If the driver won't agree to waiting, you have the option of getting out at a grocery store and then requesting another Uber from there to your resort. That would be a very minimal delay for you because there are Ubers swarming in Orlando, but you would have to unload and watch your luggage while you shopped.
There is no way to set up a shopping trip in advance through Uber. You could set one up using Uzurv, but that would involve the driver violating Uber and MCO airport pickup procedures, so I would not recommend that alternative. A ride on those terms is not worth the risk for the driver.
Another option is to use one of the grocery services: WeGoShop or Garden Grocer. Contact your hotel and see if they accept those deliveries. If so, that is probably a better and less expensive option.
